i want to install a Philips HPL-N 50W mercury bulb in a “high bay” fixture (not so high bay but i don’t know how to name it in english and i was wondering if there was a better fluorescent ballast combination to drive this lamp rather than using a 58/65W fluorescent ballast. thank you very much

A HPL-N ballast for 50w needs to have approximately 0,9H of inductance on 240v, and 0,775H of inductance on 230v, and pass 610mA of current.
Above inductances are measured from my 80w/50w, 230v/240w Philips ballast - admittedly with a cheap component tester. Use these values at your own risk.

You could go to a transformer winding company and ask them to wind you a mains voltage suitable choke for that inductance, with a couple of extra taps so you can dial in the perfect current setting as well as have options to run other lamps on it. Or, if your budget is tight, wind one yourself. But i think in Italy there should still be plenty of transformer winding companies around.

2 100w standard fluorescent ballasts in series (i know, not a super common thing to have sitting around but i just so happen to have them) will also get you to approximately 800mH and that will likely get you the appropriate current, being right between the 230v and the 240v setting.

What is wrong with your current setup? IS the lamp current incorrect, or is it having trouble starting?

A combination of F36T8 and PL-S11W fluorescent ballasts should yield correct current of 0.6A or slighttly bellow.
The 58/65W fluorescent is 0.65A, which is higher than it is supposed to be, but not severely.
Given the fact the MV are of limited supply, I would better sacrifice some light output and get longer life by driving them at lower rather than higher current.

In any case, it is matter of course to check the real current with lamp fully warmed up, just "assuming" some ballasts will yield a correct current is not enough...