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Have you tried changing the xoops_config conf_name value of debug_mode from 0 to 1?
This will set the XOOPS system up for printing any php errors it might be experiencing.
You would need to edit this value in phpMyAdmin or MySQL command line.
Once changed, you could go to the front page again too see if it prints an error, which would give you a lead to follow on fixing this.
If you have already tried this, then sorry for not helping.
Best Regards,