The train operator posted on social media that a number of people had been in contact to complain about the issue.
It warned the problem was affecting various stations, but did not mention which ones.
It said: “I’ve received a lot of messages this morning to say our ticket machines are displaying an error message at various stations. Sorry for any inconvenience this may cause.
“We’re working to fix this asap. Tickets can still be purchased from ticket office, app and onboard train.”