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Based on my observations, this problem can occur when your SQL server is experiencing difficulties or just has a heavy load in general. This in general will clear up on it's own over a very short period of time.
If the problem persits for more then a couple of hour, then ou will need to check with your host provider to make sure they themselves are not experienceing problem. Many hosts will happily restart their servers in unique situations.
If it is not a server side proble, then please feel free to come back here to see if it is just an XOOPS specific problem.
Good Luck