Experiencing the same problem, Export is no longer working and the Export popup is not responding to mouse clicks after second attempt, only ‘Enter’ key with the keyboard to get out of it (this is also happening in other popup windows in Glyphs lately - mostly plugin windows).
Hoping for a solution soon.
Sorry that it took so long. I just uploaded a new version of the SF-Symbols plugin. You need the latest cutting edge version for it to work. (You might need to remove an re-install from the plugin manager to get it).
The new plugin support some of the newer features …
Please report any issues you find.