What server platform are you using? Knowing this may help point to the solution.
This type of error is often associated with a corrupt file. It could be an .htaccess file, wp-config, or even the plugin file itself. If you have a working backup, I would restore to the last working point.
If you upgraded/updated any plugins before this error, I would disable them and see if that fixes the problem. You actually may need to do this with all of your plugins to see if there is a conflict among them.