Check that you have all of the required permissions to view the current manager page. Sometimes you can get this if for example a template dropdown can't populate because you don't have sufficient privileges.
I always start with a policy template that works and start testing subtracting privileges or adding until I get the desired permissions.
I had the same problem - upgrade from 2.2.7 to 2.5.7 (through FTP) and error 200 almost everywhere in manager.
Deleted cache, flushed session, cleared browser cache - nothing helped for me.
I deleted manager and connectors folder, then uploaded it again, and did setup again. Thats worked for me.