Is it possible that your server is in service mode (could be with newer version as they added that feature)? So you see anything else in the logs?
Just bumping up an old topic.
I faced the same issue in 8.2 environment last week, where the same error kept popping up and no matter what you did ( except NW server reinstallation) the backup server wouldnt accept any sessions.
The solution is to enable the radio button to 'Yes' in the Miscellaneous tab of the NW server properties in NMC. The option where this needs to be enabled is 'Accept new save sessions'.