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We have had the same problem, our account was suspended by our host!
This has nothing to do with Xoops, but with a reverse DNS problem in MySQL.
Start MySQL without reverse DNS and the problem is solved. You can disable DNS hostname lookups by starting mysqld with the --skip-name-resolve option. However, in this case, you can use only IP numbers in the MySQL grant tables.
I had to find this out by myself and tell our webhost, and the problem was fixed.
More info:
http://hackmysql.com/dns