Untill last Friday, I've contributed these reports to people not mounting their batteries "right" and other stuff. Then it came all over my shoot... :(
In the heat of things, I couldn't figure it out, but it seems - in my case - that it is actually the (one so far) battery which is the cause and not the mount.
I'll send this battery back, because the connections aren't right. But it wasn't the mount (even though it definitely felt like it) but the battery itself.
Just as a heads up to others experiensing this very frustrating issue.