Error log is always empty (and yes, errors are actually happening)

If I put a VatId brick in my signup form, upon submit an error page appears.
Still, the error log is empty.

I can't find an error logging configuration anywhere in the CP, nor could I find anything on the subject in the aMember wiki, nor in this forum.

Can anyone point me to what I'm doing wrong?
If you need more detailed info don't hesitate to ask.

Thanks in advance
Ric

Thanks a lot, Caesar. I had overlooked that FAQ.