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not enough info to help you here, but I'll try anyway...
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?
Using an FTP client, delete everything in /cache/ & /templates_c/ except index...
then upload a fresh copy of the modules/system/ files over the top of whatever you have WITHOUT logging in after deleting the files in the step above.
Then try and log in & update the system module.