We have a problem, condensation on a register coming from our air conditioner.
It only happens on this vent, and we are trying to fix it, but there is hardly any info on it.
Lots of water forms on the metal. Any ideas?

Condensation is a normal thing on an air conditioner evaporator, so all must be equipped with some means to get rid of the water. With split systems the condensation is collected on a "tray" underneath the evaporator (or the water->air exchanger with the chilled water loop intermediate) and drained away by piping. With compact movable units the condensate is used to cool down the condenser (it helps to increase the efficiency by the water evaporation), with the split it is usually lead to a waste water piping system and to sewage.
If the condensate goes out, it means the condensate drainage is likely clogged (dust,...). Check, whether the intake air filters (fine plastic meshes) are properly installed and cleaned regularly (they are supposed to catch all the dirt drawn in by the fan, so it won't clog the unit).
Or if that happens with recently installed unit, the complete unit could be just misaligned (it should be positioned exactly horizontally, so the drain hole is really on the lowest point; otherwise the water can not drain properly, it overfills the pan and then get spilled around).

Condensation is a normal thing on an air conditioner evaporator, so all must be equipped with some means to get rid of the water. With split systems the condensation is collected on a "tray" underneath the evaporator (or the water->air exchanger with the chilled water loop intermediate) and drained away by piping. With compact movable units the condensate is used to cool down the condenser (it helps to increase the efficiency by the water evaporation), with the split it is usually lead to a waste water piping system and to sewage.
If the condensate goes out, it means the condensate drainage is likely clogged (dust,...). Check, whether the intake air filters (fine plastic meshes) are properly installed and cleaned regularly (they are supposed to catch all the dirt drawn in by the fan, so it won't clog the unit).
Or if that happens with recently installed unit, the complete unit could be just misaligned (it should be positioned exactly horizontally, so the drain hole is really on the lowest point; otherwise the water can not drain properly, it overfills the pan and then get spilled around).

It's not that the water from the evaporated is going down to the vent, It's rather that the vent air for that particular one is chilling the register and condensation is forming on the metal part. The evaporated water from the evaporator is sent down to the outside, in a pipe. It's not the evaporator's fault, it's the register.
The current thing we're doing is my father went into the attic, and he saw that part of the ducts was completely unattached, which we assume is the fault of the nutty air conditioner installers, and so a lot of the conditioned air was actually cooling the attic  , so we attached it with some adhesive, but it takes a day to dry, so in the meantime, we are using the portable air conditioner and a permantly attached ancient vent fan (before air conditioning, this house is that old) and slowly cooling the house down. I hope there will be no more condensation. Sorry for the long story, I guess I got worked up by the heat.
Is the fact that the register is not painted a factor?

Does the watter appear while the air is flowing or when it stops ?

If when the air is flowing, the air coming out of the register is blowing away the room air, so the moisture must be one that comes with the AC air

If when it stops, then its moisture from the room air. Are there any sources of moisture inside that room ?

Does the watter appear while the air is flowing or when it stops ?

If when the air is flowing, the air coming out of the register is blowing away the room air, so the moisture must be one that comes with the AC air

If when it stops, then its moisture from the room air. Are there any sources of moisture inside that room ?

It only happens when air is flowing, which is most of the time in the day.
But isn't AC air not humid?
And the water that forms is away from the air, which is the cover around it, and there is no condensation on the louvers.
Edit: Should I measure the humidity coming in and out of the air conditioner? And is the fact that we have the "Fan On" option on important? It just keeps the air conditioner fan on at a low speed. And the vent is located in a small bathroom with a shower in it, but we put the bathroom exhaust vent on when we take a shower.
The evaparator water pipe just leads outside to near the heat pump.

Gotta love a muggy southern summer, where you can wash a table with the condensation from an ice cold beverage!

The air has a lot of evaporated water in it, but usually during the day is below dew point.  However, when you pass the air trough the evaporator, it can be cooled below the dew point, causing the condensation that must be drained away.  When the air leaves the air conditioner and heats back up in the room, there's no replacement for the moisture left on the evaporator coils, so that air is "dryer"  However, the air that's cold just as it's leaving the air conditioner is still saturated.

The fan blowing will cause a little bit too.  When the air conditioner is running, it will be chilling the metal vent.  When the AC shuts off, the first part to be heated will be the evaporator, so the cold metal vent can cool the air, causing condensation.

If possible, replacing the metal vent with a plastic vent may help, as the plastic won't "store cold" as well as the metal one does.

Gotta love a muggy southern summer, where you can wash a table with the condensation from an ice cold beverage!

The air has a lot of evaporated water in it, but usually during the day is below dew point.  However, when you pass the air trough the evaporator, it can be cooled below the dew point, causing the condensation that must be drained away.  When the air leaves the air conditioner and heats back up in the room, there's no replacement for the moisture left on the evaporator coils, so that air is "dryer"  However, the air that's cold just as it's leaving the air conditioner is still saturated.

The fan blowing will cause a little bit too.  When the air conditioner is running, it will be chilling the metal vent.  When the AC shuts off, the first part to be heated will be the evaporator, so the cold metal vent can cool the air, causing condensation.

If possible, replacing the metal vent with a plastic vent may help, as the plastic won't "store cold" as well as the metal one does.

Thanks! We will have to try the fan on auto tomarrow.
We have to let the adhesive dry.

Update: The air conditioner is on!
We assume the sealant is dry, so we tried it out, and we will have to check it.
But we will have to see if there is condensation on the register.

Locate the condensesation line out side plastic pipe that drip water outside your home, most likely it clogged
Cause of insects or alagee in the drain line.  I would get a wet/dry shop vac and vacume out drain line, this should be done every season during first couple of days running your system.

Locate the condensesation line out side plastic pipe that drip water outside your home, most likely it clogged
Cause of insects or alagee in the drain line.  I would get a wet/dry shop vac and vacume out drain line, this should be done every season during first couple of days running your system.

But that can't be the problem because it only happens on this vent and forms on the vent itself.

Well, if the condensation drainage is clogged, the AC may be just spitting the water out of the vent...

The output air is for sure too dry to cause any condensation - the coldest spot was the evaporator and so at that point all the water has condensed, so the dew point of the output air is the evaporator's temperature. And because after that it can only warm up, it can not condense the water anymore.
Of course, if the water stays at the evaporator and so gets picked up by the air blow, it remains wet.

Well, if the condensation drainage is clogged, the AC may be just spitting the water out of the vent...

The output air is for sure too dry to cause any condensation - the coldest spot was the evaporator and so at that point all the water has condensed, so the dew point of the output air is the evaporator's temperature. And because after that it can only warm up, it can not condense the water anymore.
Of course, if the water stays at the evaporator and so gets picked up by the air blow, it remains wet.

So it must mean that the air surroundung the vent is too humid, but how do we fix that?

No, the room air have nothing to do with it, as the air coming out of the register blows it away

There is some source of moisture inside the A/C system - Clogged drain or some other water source leaking into the ducts somewhere. Then this water is evaporated, carried with the air, and condenses in places inside the ducts. Maybe the ducts to this room are less insulated so the water does not condense on them, but continues all the way to the room

No, the room air have nothing to do with it, as the air coming out of the register blows it away

There is some source of moisture inside the A/C system - Clogged drain or some other water source leaking into the ducts somewhere. Then this water is evaporated, carried with the air, and condenses in places inside the ducts. Maybe the ducts to this room are less insulated so the water does not condense on them, but continues all the way to the room

That cannot be it, the louvers are directing the (cold) air away from where condensation is forming, and there is a lot of air space there, and the conditioned air is at a low speed there.
I tried to measure it with my weather meter, but the humidity just goes really low then really high.
The register has a lot of space inside where the air could maybe hit the metal and cool it. It even feels cold.
Where could we add insulation, maybe?

Measure the humidity in the room when the register is blocked and in various places inside the duct that comes to the register

When condensation is forming (at changes of air temperature) the moisture sensor may behave eratically - The water content in the air becomes non uniform, so what the sensor will show is a hit and miss