InfoPath form server error messages are misleading.
During development of an InfoPath form, I would post it to MOSS server and access the form. The form would start to load and then generate a misleading error message pointing me to the windows event log for details. In fact, no message was written to the windows event log. Rather, the message was sent to the MOSS ascii diagnostic log. You can track that down via central services administration.
You need to be quick on your feet. MOSS likes to write to the log file, frequently and verbosely. This can be trimmed but the default log writing behavior is "everything as quickly as possible".