Is it possible you have a GDEF in your manual feature code? Take a look into File > Font Info > Features > Prefix. And what is at line 853 in features.fea?
Hi, well this happened once again.
As I followed your advice, looking at Prefix, " table GDEF { " starts at line 130 and ends at line 132. No more lines.
Glyphs does not generate GDEF code in the prefix. Any particular reason why you put that code there? Or is it a leftover from a conversion? Have you tried removing the prefix and exporting again?