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Very unsure on what that means as to see a correct install of Xoops, index.php is the main page...
so if you don't see the main page then that isn't installed correctly?
If on other hand you mean on switching php debug on in admin, you found the front page gone due to an internal error happening...and that causing an overload that should still present the reason why, the error would occur if php debug is on....
So suggest checking if XOOPS is installed correctly, to begin...just make sure in your mainfile.php the setting and url's are correct 1st...if they are and you still get a blank index.php...
Try turning off debug, if you can get to admin/index.php...if not Try re-uploading the files....