I have a Glyphs file which I can export, but Glyphs 3 can not open the exported OTF/TTF file. This error is shown when opening the exported font file:
I could reduce the bug to this minimal case: Test.glyphs (1.4 KB)
The problem seems to be a substitution rule of the following format:
sub a' [x] [x y] [x] by b;
Glyphs 2 can open the files as exported by Glyphs 3.