Unable to use it, unable to uninstall

till 3.1.14 or 15 , it worked well.
Every update since, denied me from logging on to wp-admin as if user/pw were wrong. Each time I solved it by restoring site backup.
Now I tried 3.1.19 , enabling it gives an awful error(basically kills the site) , trying to uninstall to start with “clean sheets”
but that fails with “Deletion failed: Internal Server Error”
Please help.