It should not matter if my theme is up to date or not. What if theme development stops because the author quits his business…? So I don’t think that’s the issue. I think it’s strange that I could change the labels and after updating SLP I can’t change them anymore, and now I am stuck because my client doesn’t want that labeltext (plus: the site WPML now, and I can’t find the labelinput texts anywhere in string translation, but that’s another issue).

So I think you should have a solution to this problem. I think your product has an actual error, since I could make changes after buying it, and now, after updating, I can’t.