Okey, so I found the thread I mentioned before: https://www.dual-boxing.com/threads/...ons-in-thread)

And the important bit from said thread:

Quote Originally Posted by Vociferate View Post
I've had to do some tedious work, but I have found what was causing an issue.

One of the lines of code in ElvUI was conflicting with ISBoxer. Don't ask me how, I have no idea. But the maker of Skullflower UI was able to have a look. It was something to do with the unitframes. :O
Don't know if Vociferate still reads the forums, but I guess if he does he might remember more about this.