Hi Lars and
to the forum.
Unfortunately it sounds like your controller has failed and will need to be replaced. I suggest you contact your supplier and explain the fault and they will hopefully send you a replacement controller.
Accessing the controller should be relatively easy once the wheel has been removed from the bike.
If you watch
Gary's video you will see exactly how it's done.
The circlip that secures the cable against the axle can sometimes be a bit awkward to remove where it fits into the tiny holes in the axle. All you need to do is use a pair of pointed tweezers or a small thin flat blade screwdriver to gently ease out each end of the clip in turn:
Here is a picture showing the clip holding the cable nice and snug into the axle's recess, to keep it well away from the centre of the rotating cooling fan/disc mount assembly to prevent damage from chaffing:
Alan