This happens when I updated Glyphs to Version 2.6.2 (1248) and exporting fonts with Autohint checked.
There is another thread with the same problem. Can you send me the .glyphs file?
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Found the problem and fixed it.
I am getting this error after I have upgraded to glyphs Version 2.6.2 (1254)
Version 2.6.2 (1254)