Didn’t really understand, can you explain it a bit more? Sorry 😅 About your warning is it dangerous to measure a 230V current with my multimeter (with precautions, gloves mainly)

Depending on the meter and your approach it can be either safe or lethal. The more I think about it, the more I’d recommend that you just don’t do it at all. Working on 240V installations under voltage, that is something we teach electricians to do safely. Besides the training it involves some special and rather pricey equipment.

I cannot stress this enough, if you mess up, you will hurt yourself with enough severity that the can easily be fatal. If you don’t know what you’re doing, messing up will be exponentially more likely.

Desolder the transistor, does the rest of the board light up? If so replace the transistor. Otherwise, get trained assistance on person or replace the UPS. I’m all for learning electronics, but not on a live UPS.