Where should I head to for eliminating this error:
The Masters render perfectly, the Instances show these errors.
Have you tried clearing your font caches?
https://glyphsapp.com/tutorials/eliminating-font-cache-problems
That looks indeed like a corrupted font cache. Did you try to open this font in Glyphs again to see it it messed up, too?
Something like this can also happen if you use the Add Missing Symbol Glyphs parameter for a TTF.
After deleting the font caches and restarting, then exporting anew, the instances were still mixed up. But after deactivating Add Missing Symbol Glyphs, then deleting font caches and restarting and exporting, it worked.
Thank you for your support!