THis is what I did:
Swapped the batteries, it worked.
Checked the wiring, wasn't anything wrong.
Reconnected the ''faulty'' battery, same problem.
Opened the battery on the charging side and found this:
See the attached pictures.
After fixing this, the bike worked perfectly again.
Conclusion, it had to have been the battery but it's really weird and I don't understand as to why a faulty charging cable could cause a regen braking error like this.
Also my MP hasn't got a horn that beeps as it was the second or maybe even the first MP that shipped out of the GM factory.
But thanks for your quick reply and to my suprise you still knew my name.