Assuming you start X11 by either XDM or ‘startx’, then fvwm should log to the file ~/.xsession-errors, by default. If you use other display managers, you’ll have to treat them as a special case, and configure them separately.
hmm, ok, that might be the problem. I’m using qingy instead of xdm/startx. But I was going to drop qingy anyway once I configured xdm better. I’ll see if that solves my problem. Maybe the logs are in the qingylogfile