Forum OpenACS Q&A: Re: First requests fails: invalid command name "rp_filter"
Looks like your filter didn't get registered. Strange that there's nothing else in the error log. You could try turning on debug parameters mentioned in rp_debug (http://openacs.org/api-doc/proc-view?proc=rp_debug&source_p=1&version_id= ). That might shed some light on the issue.
Hope that helps,