As stated in the subject, I accidentally supplied 24V instead of 12V to my APU2C4. Of course it got burned out (figuratively).
After a quick view on the schematics and a couple tests with a mutimeter I discovered the zener diode D21 was broken (SMBJ14A). Being it easy to replace with a soldering tool and to find it on electronic components shops, the fix was rather simple. Just wanted to share my experience here FTR for people not so used to read and interpret technical drawings.
BTW, I think it would be really good if the APU2 could be powered directly with 24VDC. It's a standard voltage level in industrial facilities, therefore pretty easy to find in such places (as opposite to 12V).