EDIT: My add-on seems to be breaking the unwrap-reset operator. I’ll look into it and then post what caused it.
I can’t imagine this hasn’t been asked before, however, I can’t find any topic about it.
It happened some months ago that Blender 2.78 started having problems with unwrapping with the reset method. It used to map all quads to squares, now I get some weird lines.
It still works in older versions but it also seems to be tied to files. Opening a file with that problem in an older version also breaks the reset UV operator.
This happens to me on all computers that I have and I also heard that a friend has the same problem. I might be missing something, however, I hope that one of you has a solution for this problem.