Thanks for the reply. I have received back 3 of the batteries repaired from my dealer, they also said I should stop using the tool that causes this issue. I'm about to send back one of the string trimmer motor units to my dealer for repair, but I assume they'll do a lousy switch of the unit and no real fix. It's not promising that you got the exact same issues with a repaired/replaced unit.
I've also had contact with Milwaukee EU directly and they only hid behind "this tool is developed for the High Output batteries, try those", even though that info I cannot find anywhere on their website/manual etc. On the Milwaukee EU website they even sell a package of the string trimmer + 5Ah (non high output) battery, so that don't make any sense. In the manual they have a "recomended battery" section which states the normal + high output batteries. I've send that info back to Milwaukee, but they haven't replied for a week yet. Let's see what happens. At least everything is going to the service center asap.