Hi Paul, It sounds like your process has been updated after this ticket was put into the status With Customer.
The reason why you are getting the error is because your process will have an automatic action called Start the Clock after Manual action Back from Customer.
The only way out is to change the process temporarily so you have a route to another status that does not include the automatic action called Start the Clock.
I suggest you create the route from With Customer to Resolve action, activate the process again, deal with this ticket,and then delete the route again.
Obviously - test this in your test enviroment so you're comfortable doing it in production.