I have the same error message, and I'm fairly certain it's not related to the access rights of any particular remote user, but initially I thought: kudos to the comment about this involving the Console session, because that's what I'm doing.
I have a half dozen Windows Server 2012 R2 VMs I am connecting to, all on a workgroup (no domain, so no domain admin accounts). I am logging onto each VM as the Administrator user (this is a testing / development environment so I'm happy with this approach). Windows Firewall is disabled for Domain/Private/Public, and no third party firewall or AV program has been installed. I am logged onto my local PC as a workgroup user who is a member of the Administrators group on my local PC, but which does not exist on the VMs. UAC is disabled in all locations. I am connecting to the console session on all of the servers. I am connecting to the console session on all servers. I am also connecting to these servers behind a NAT-enabled router, so I connect to 6 VMs using one IP address, but with a different port for each.
1. Running RDCM as an administrator changed nothing (as expected, since I'm already a local administrator).
2. Connecting to a server non-console ("Connection Settings" tab, "Connect to Console" unchecked) did not fix the issue.
3. I read that the following registry key may not exist (but it did, so not my fix):
"HKLM\SYSTEM\CurrentControlSet\Control\Terminal Server\AllowRemoteRPC” DWORD value should be 1.
Hopefully this info helps someone in their quest to find a solution. I am able to log onto a group of servers and disconnect from the same group of servers, but it would be nice if logging off of those servers also worked. It seems the issue is simply that the user I launch RDCM with on my local PC is not a user on the remote PC (no domain), and the request to log off is being sent as the local user of RDCM instead of the user which is logged onto the remote PC. So for those of us without domains, maybe there is no workaround?
Ha! I didn't try the last solution! This worked for me:
"HKLM\SYSTEM\CurrentControlSet\Control\Terminal Server\AllowRemoteRPC” DWORD value should be 1.