Well how ironic - just had this again and to enable remote panel you need to enable HTTPS. You can't enable HTTPS when there is a job in the queue. Head and wall spring to mind.
Also - this isn't an A4/Letter issue - this is the old 116-324 font code problem. Had to get the customer to reboot the machine 3 times whilst trying different fixes - a little embarassing tbh.
Technically you can't, but if the firmware is up to date, you can use Remote control panel, and in any case, you can have it simply automagically switch between Letter and A4 with no intervention.
EWS > Home > Quick Links Log in as admin to see
EWS > System > Defaults and Policies
I seem to have a lot of customers who manage to knock the paper tray settings when loading plain A4 paper. They either change the type to 'Custom Type 1' or the paper size to Letter.
Remotely this cannot be solved if a 'System' print job is in the queue (like the start-up page).
This means talking a customer through placing paper in the bypass tray before we can change the paper type or size to 'Dedicated' in the Device settings.
You can do no setting changes with this device if a job is stuck in the queue.