While doesn’t connect to Windows 2012 R2 server via RDCM, can connect via MSTSC /v:
May receive an error about *3334* for that server but can connect to other servers in network.
Remote Desktop Connection Manager can only open 6 sessions at a time on Server 2012.
For a try, please check below settings and let us know the result.
1. On the system where you run RDCMan, Run dcomcnfg.exe
2. Expand Component Services, My Computer, Distributed Transaction Coordinator.
3. Right click on Local DTC and select properties.
4. Click the Security tab
5. Check the Network DTC Access box.
6. Under Transaction Manager Communication, select the No Authentication Required radio button.
Click the Apply and close windows.
MSDTC service will be restarted and all dependent services will be stopped.
Applications using MSDTC may need to be restarted to use the new settings.