After upgrading the database from 1.5.4 to 1.5.5e I got the famous Server error 500 when opening the admin section. I spend some time googling for the answer. After some trial and error it turned out to be the missing language files. After adding the language pack all worked fine again. Maybe this solution can be added to the list of possible causes for error 500.

This is an issue that will be happening on every database upgrade when the infromation in the database differs from the actual site.

Maybe that this item cab be of help to someone .