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Hi Jack,
the only time I have seen this before, was when the MySQL user didn't have the correct rights for the MySQL database. For instance, the MySQL user didn't have access to the /tmp/ folder (not in XOOPS, but in the server setup).
My suggestion is: check the database settings and see if other applications work. Try with phpMyAdmin, for instance.
Also, is the banner system you have above the header a script? Try to disable that, as it may be interfering with the XOOPS site.
Good luck!
Herko