That should be fixed. Can you try to get the latest version of the RMX tools?
1.1.17 currently is the latest available version of RMX.
I get the same error.
Correct, I am on the latest RMX build. Tried reinstalling to no avail.
Another piece of info I discovered @GeorgSeifert : The error only happens when using RMX in Font View
The problem is being worked on. It should work in the latest cutting edge version of Glyphs and Tim should be in the process of publishing an update.
Thank you, both!