Error 500 after upgrading to wp 3.1 only in wp-admin

Hi everybody! i have a trouble with 3.1. After i upgraded my wp 3.0.1 to 3.1 automatic, the rest of the web works fine, but i can´t access to http://wwww.mysite.com/wp-admin i get error 500.

I have tried with deactivate the plugins, add to 64 mb the php.ini but doesn´t work….

any suggestion? thanks in advance