by Ken Task.
Error 500's are kinda a general catch all and not very descriptive of the problem. And you've not given anyone much to go on ... server local, remotely hosted and shared (provider), locally hosted in a virtual environment, or locally hosted as a stand alone ... operating system, etc..
But ... check permissions on the moodledata/temp folder. It is there that Moodle must be able to access a folder called 'backup'. The web server user must be able to create folders/files in that directory.
Check for existence of that folder and it's contents. Anything?
Could temp files be created in the operating system temp folder?
Since it's an upgrade ... upgrade from what?
'spirit of sharing', Ken