Error - 'Unable to retrieve version of the server'

After some assistance with an issue which has recently appeared in our NBU environment.

We have two master servers, running Windows Server 2008 R2.

When logged onto MASTER1 with the NBU administration console running, I can no longer switch master server by right clicking and selecting'Change Server . . .'

When I attempt to connect to MASTER2 I get the following error message as below and vice versa from MASTER2 to 1.

Any suggestions as to what could be causing this issue?

Thanks in advance.