I turned the site off and turned on MySQL debug. This is the problematic line:
SELECT forum_type, forum_name, forum_access, allow_html, allow_sig, posts_per_page, hot_threshold, topics_per_page FROM xoops_bb_forums WHERE forum_id = 3
Error number: 1054
Error message: Unknown column 'forum_access' in 'field list'
Obviously, something happened to the db. What happened? How db structure can change? And, how to fix it?