What kind of error is this? It seems, that there is a 3rd path in this master. But it isn’t.
Glyphs 3.1.2 (3151)
That is probably an empty path, do you use the Magic Remove plugin by any chance? That leaves empty paths.
Cmd+T (Tidy up Paths) fixes this.
ahh, simple solution. It drove me nuts already. Thanks a lot!