Sounds like the alarm unit, on the controller there is a yellow wire which needs 60v (70v when battery is full) to switch on, this 60v comes from the alarm unit (basicly electronic lock), see foto of the connector on the speed controller, I had to connect my kelly to this wire to get it to start together with the original controller. If you put 60 volt on this wire the bike will bypass the electronic lock (alarm)

Thank you arjan!

I thought before I'll try that, I try to check the circuit of the alarm system. So I disassembled it today and made a view checks of the circuit board and the components on it. I found a diode which can be the problem. It seems that it is shorted in both directions.

I think I have to unsolder it, to be sure. I wonder whether that could have been all or whether more components are affected.

But.. you really mean an Alarm Unit on the Controller? Is there one? I'm talking about the separate Alarm-Unit which is located a little bit under the Start-Button.

I have now unsoldered the diode. It's a Schottky Diode SS210. I measured it again, it is definitely broken.

I wonder whether the 70V reverse voltage given in the data sheet of the diode is perhaps too low, since my battery reaches 70V when it is charged.

I will order a new one. 200 pieces of the diodes cost 7.95Eur +5 Eur shipping, that's too many for me. But maybe I need them every 1000km 😉
I will let you know if the Soco starts again with a new diode.

These diodes are normally used for inrush current from coils when switched off, they are in every car ecu, here they are probably used when switching off the controller and other consumers and to protect it from damage to allow the inrush current to dissipate to ground (minus)