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It's possible that you did follow the instructions with the Protector, in case you have it, or there is an old module that is not compatible.
If you have access to something like phpMyAdmin, find the table "modules" and set for all modules except the "system" module the value for "isactive" to 0, and then try again.
Do the upgrade, and if it works, then go to Admin, and activate the modules one by one, and after each activation, check the front if it works. Once you figure out which module was causing problems, you can update it to the latest version. Well, you should update all of them.