You should not ever upload your user.config from your local machine to the server, but you should ALWAYS use the new Web.config that ships with mojoPortal, so when upgrading you must always use the new Web.config file. You should backup your old Web.config (really you should back up your whole site and database) before upgrading, then you need to restore customizations from your old Web.config file to the new one (such as the machine key setting especially)
If you still get errors after that try downloading the file /Data/currentlog.config and see if any error details are there.
Hope it helps,
Joe