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Hi, welcome on the XOOPS forum!
It seems an old server to me, because it is still PHP 4 and MySQL 4, while nowadays version 5 is regular. But that is not the problem.
I assume you have installed XOOPS in the subdirectory forum of your domain. Can you verify that in your mainfile.php (located in /forum/) your XOOPS_URL and XOOPS_ROOT_PATH are both correct and ending on /forum ?
Did you set all the required directories to writable, as described in the instructions?
Is the file /forum/Frameworks/textsanitizer/module.textsanitizer.php present?
Did you verify that all files are present with checksum.php?