Issues with speedometer

[vv3]

Hi!
I have a 6 yeard old (or so) Rad Rhino EU 250W, pre-facelift.
Its having issues with the speedometer, which is also affecting the electric ride assist.
The speedometer sometimes work fine and all of a sudden it starts to show random numbers (0 or 58 or whatever) and is randomly jumping from number to number. When the showed speed is high the electric motor does not work, as it thinks you are going too fast anyway.

I took it to the bike shop and they could not find the fault. I also tried to find a connector which would be coroded. Found one, cleaned it with vinegar + alcohol, but the fault did not go away.

Any suggestions?

Thank you!