It'll make things easier if you post your XOOPS / php version, what page displays the message you posted? can you access the site at all? is this in Admin Only?
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* XOOPS installation md5 checksumminig script
*
* This script allows you to check that the XOOPS system files have been correctly uploaded.
* It reads all the XOOPS files and reports missing or invalid ones.
*
* Instructions:
* - Upload xoops_md5.php and xoops.md5 to your XOOPS documents root
* - Browse to xoops_md5.php
* - Re-upload missing/invalid files