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Most of the time this is related to wrong configuration of Trust Path, or missing module files in the /xoops_lib directory
1) Make sure that you have the correct Protector files in /modules/protector, and in /xoops_lib/modules/protector (as in the 2.4.5 package)
2) also, make sure that the mainfile.php points the "trust path" to the /xoops_lib directory, and to your old trust path directory that you used before the upgrade
3) If you use "altsystem" module, make sure that the altsys files that are designated to be in the trust path, are in /xoops_lib/libs/altsys directory
3) If this is all OK, and you're still getting empty page, go to phpMyAdmin, and in the table "xxx_modules" table (where xxx is your prefix), set the "isactive" to 0 to your modules (except the System module). Do it one by one and test if the white page goes away. This way you'll find out which module is causing the problems. If it's an older module, you might need to updated it to a newer version.