I'm just quoting the manual, as I have no direct experience with it:
Problem:
Error symbol appears on the display [1-20].
Possible causes:
The machine electronics have detected a serious error.
Remedy:
Switch off the machine by pressing the on/off switch [1-8] and allow to cool for a few minutes.
Switch on the machine again by pressing the on/off switch [1-8].
If the error symbol appears again, contact after-sales service.
Did you have any other error conditions before it started doing this? Bad adhesion, loss of contact, overheating, etc?