If you upgrade Release Management, you always need to update the Client. Up to this, fair enough.
But you are going to start getting errors like “The account running the TFS build service needs to be added as a system user in the Release Management Server” in your build.
What you might be missing is that in order to allow communication between the two you also need to at least open the client, and if you keep getting errors you might also want to remove and re-add the Release Management URL into it.
This operation will recreate the authentication tokens and you can use VSRM again.