I got it off....finally! Had to use a vise (or vice?) we had in our forklift garage at work, but it came off. The ND is in the bin now, but I don't care...it's easy to get a new one here, the adaptor rings has to be ordered from England and takes forever.
PS: The problem I had was that I use a Lee wide angle adaptor ring. It screws on the lens in such a way that it kinda wraps backwards over the lens to avoid vignetting when attaching the filter rig. When I then screwed it onto the ND filter the ND filter was kinda cradled within the adaptorring in such a way that when you looked at them from the side, only about half a millimeter of the filter threads on the ND was visible. That meant using lid openers and such wasn't possible, that's only doable of you have two filters attached to each other or similar. But luckily we had the right tools in our terminal here.
Anyways: Thanks for all the good suggestions! Really appreciate it. 