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Guido:
Error 145 usually means the table needs to be repaired.
With phpMyAdmin, it's easy to check and repair tables. From the page that lists all the tables, select the checkbox for the xoops_priv_msgs table on the left, and then in the menu at the bottom, select "With selected: Check table", and then "With selected: Repair table".
Does that help?
You might as well check all the tables while you're there. You may want to do the Analyze and Optimize operations too.