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I am not sure that it is your server causing the problems. I would first start off with:
1. Create a directory called xoopstest
2. Create a NEW database called xoopstest
3. Username and password xoopstest
4. Go to the xoopstest directory and install the new test xoops
5. See if the issues still happen.
I am not a huge php programmer, although it seems as if (Now I am not 100% here) there is a database record lock problem.
In ASP I would get a problem like that if there was a locking problem, or if there was a record that did not have write permissions (usually record locking, or sharing)
If the test XOOPs site works without a problem then that is the problem, if you still get the same errors then accept my apology for waisting your time.