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This is a Javascript related error and could be caused by either not having Javascript enabled on your browser, or due to file corruption during upload of your files.
If you are sure it is not your browser, go to your local copy and re-upload /html/includes/xoops.js to the same directory on your server. If that fails, download another copy from XOOPS unzip (unrar, untar), look for the file and repeat. It may have been corupted during the original download or decompression.
These are the immediate problems that come to mind and others may have alternate solutions. My suggestions are just ways of troublshooting to eleminate the possibility and have the ability to look else where with reasonable confidence.