I'd pesonally say that there is something broken in the config. You might check with httpfox (Plugin for firefox) or similar stuff what the delivered content-type is in the http header. But I assume it is plain text and not text/html.
Thus any config setting for serving either the default or custom error pages must be wrong.
Sorry to be so unclear.
You might also check the access / error log while reproducing this behaviour, maybe this might give you a hint as well.
did this beahaviour just appear for specific vhosts and / or out of a sudden or since the first setup?