Log on to your Medilink server.
Start Medilink Launcher but do not Login yet.
Wait for it to say MDS Subscription Active in the bottom corner.
Then click Login -> Medilink
Once in Medilink proper, double check File -> Registered Mode is ticked (and/or simply re-tick it).
Once the Server is fixed Workstations should automatically pick up the Registration from the Server, although you can also do the same as above.