I decided to bleed the rear Tektro brake and after releasing some air filled up with some more oil (just a little bit). After that the error 25 came in and the brake light was constantly on.
Removing some oil, then some more oil, then some more oil – didn’t work. The error and the break light was still there.
Disconnecting the rear brake sensor worked – the error went away. However that was not much of a solution to the problem.
Since we have no manual I had to guess. Then I thought – if the error came in due to, possibly, higher pressure in the system (as if I were breaking) then this must be adjustable, or should be.
It is – on the lever, behind the oil tank, you have the break sensor with the cable which goes to the controller. UNDER the break sensor there is a hex screw size 2.0. When I unscrewed the screw a bit, the error went away. After applying the break again – it came back. Slacked the screw a bit more – now the error is not coming back and the break works and so does the break light and the motor cut-off.