For anyone who's interested in the details... the "could not insert forum post" arises when there is a database error. One common cause of a database error is not having a connection... but this would cause other things to fail. The other common reason is this HTTP_REFERER check. As a security precaution, whenever you submit a form, XOOPS checks whether the HTTP_REFERER is the same site, and if so, allows update/insert/etc... database privileges. If the referer is not present or different (often due to a firewall), then the database is given only permissions to do 'SELECT' queries. Any other query results in a DB error, and hence the error message.
Thanks for the correction.