This is caused by a license issue - see below Newfielt info and resolution details in last paragraph.
From Adam at NewField IT:
The cause of the issue was the licenses, with Equitrac version 4, if you have a system where the licenses were shown incorrectly i.e. Available (0 or LOWER) vs Consumed (>1 or HIGHER), every so often, quite infrequently & without warning, you will get machines that show "Communication Error" or simply don't process cards on swipe or alternate logon attempts.
This is because for that amount of time, before the server was rebooted and it came back online, the machines lost their license (or seat) & because it shows 0 available, it won't be able to pull a license for any machines that try to connect. If you did it the other way round i.e. reboot machine, then it would be fine because the server is still holding the seat for that device which will allow it to reconnect. The Luton server therefore going off & lack of what the system thought were available licenses, meant it could not recover those seats for the 4 devices in Luton.
Most Discovery/ConnectKey devices would show "Not Licensed", but in this instance it would simply just disallow that machine to Communicate or retrieve any configuration data from DCE, hence the error message "Communication Error", it just blocks it.
The fix was to correctly update the Equitrac license table, which can sometimes get "stuck" (the very technical term), so it didn't show the correct number of Available vs Consumed, and force this to update the Main CAS table(s). After this a restart of the CAS Service, clear DCE cache on Server & force cache update, resolves the issue & allows the machine to re-pull their seat licenses correctly.