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Turn on MySQL debug and you'll probably see a 'can't open database.tablename.myi' error in red or something similar. If this is the case, then there is something wrong with the MySQL configuration, probably the MySQL user that owns the tables of your XOOPS site doesn't have the correct permissions to write to the database server's /tmp/ folder. Anyways, a lot of technical talk, but basically it is a server configuration issue. Contact your hosting provider or your server admin.
Herko