The error can occur in multiple places in the application. When you do the first thing to consider is Servers.config. Check for the ServersTemplate.config with a known good ServersTemplate.config then you can:
-----If on server and ppart-----------
1- Rename the Servers.config.
2- Restart all the Services.
3- Verify that the Servers.config was created. If not you may want to launch DBviewer and see if that recreates it.

----------If on a client ------------------
1- check for the ServersTemplate.config in the client folder with a known good ServersTemplate.config
2- Rename the Servers.config.
3- Start the application and this should create another Servers.config. If not report this to Support.