Code 500 is a generic code meaning that the server has detected some sort of error.
On the limited information that you have supplied it would appear that you can't get to the admin screen. The oly alternative that I can suggest is to go to dokuwiki/lib/plugins
and move the directory for the last plugin. When I've had a similar problem I simply moved it down a level so that it isn't found.
How do you back up the rest of your machine? If you have an established backup regime for the system it ought to back up DokuWiki happily. There are no permanently open and locked files.