You know, I'm not sure; the OS shouldn't matter; and UBB6 is compatible with PHP5/MySQL5...

It is likely an ownership problem; simply have them chown the web directory containing the ubb files to the proper owners and check permissions on the files and you should be back in business.

As an FYI, I'm running a UBB.T6 board on MySQL/PHP5 without problems (in a test environment)

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