– I am attached to the physical desktop of the Mint machine.
– I believe the numlock was enabled on both the client and server hosts.
When I checked the server side after the problem occurred, I could see that the numlock light was flashing rapidly (that was also mentioned in the forum post you mentioned).
Not sure if it is related, but in the past, when running Ubuntu 12.04LTS on the server side, I would often notice that the status of numlock on the server would get out of sync with numlock on the client. For example, the keyboard on the client would have numlock off, but the server was producing all caps. I’m not sure what triggered this – it would sometimes happen several hours after the client was connected (I often work remotely for an entire day).