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Two possible fixes for this.
Log into your site VIA FTP and delete your /templates_c directory files.
Second, and most likely, you may have to kill any open sessions found in your MySQL tables.
To do this, go to your database VIA phpMyAdmin. Load up the database you have used for your site and look for a table called xoops_sessions (assuming you have used the default prefix).
Once there, click on browse to see the contents of the table. You will see here a list of IP's that are currently using sessions on your site (this happens irregardless of your preference settings in XOOPS Admin). Go to the bottom of the page and click on select all, then delete all table. Once asked for confirmation, select yes.
To be sure, back up your database first, in case by some unknown force of the universe, you happen to delete the wrong tables.
This should take care of your problem.
XOOPS Community Support