[xmonad] xmobar crash analysis

Hi all! I hope it's ok to ask xmobar questions here, it fills related enough. Frankly, I don't know if my problem is caused by xmobar or xmonad. Every now and then, maybe every other day, my xmobar crashes. It just disappears, leaving a gap where I can see my background picture. The only way to make it reappear is to log out and in. Restarting xmobar from a terminal (same command as in my xmonad.hs) starts the process, says that "Fields missing from config defaulted: overrideRedirect,persistent", but the bar does not appear. Mod+q doesn't help. What I wonder is how I find crash and debug info about this. /var/log/syslog doesn't mention anything about xmobar, xmonad.errors is empty. What is going on? Thanks, Gauthier