After contacting the support I received a replacement battery pack sent what worked well with no problems, but a big plus for the support.
However, this replacement battery fell out after about 2 1/2 days as the first, in explicit compliance with the operating instructions in the manual.
After renewed contact with the support I received my money back, but again a point on the support.
I have no idea what the errors were, but because the battery under these circumstances is completely unusable, I decided for another manufacturer.