Maybe its the hole into which the pushbuttons fit. Maybe there is a fault inside.
From the manual
"If error message E011 is displayed, a button may have become stuck. Press each button individually and check that they are all clean. If the error message remains, please contact the after-sales service"
I find a drop of WD40 on a cotton bud is really good for getting grease and crud from inside little holes like this.
xxx, January 2017
