Internal Server Error The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator to inform of the time the error occurred and of anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Which is why I always make a copy of the site and do it there - because now the site is inaccessible. Not only did I have to restore the previous plugin code, but also the db. And it wasn't enough to just restore the mailbox tables - I had to restore the entire db. Again.
Same thing happened with the Forum plugin and a few other core plugins.