Glyphs 3 features preview broken?

A small font with only a calt feature compiles fine but does not want to preview. There is a red exclamation mark in the bottom left corner. The same file in Glyphs 2 previews fine. (Build 3046)

Try the latest cutting edge version. For me, the red exclamation mark has been fixed in 3053. See also:

Suddenly, I have the same issue in 3.0.2 (3058). The problem persists, even after I delete all the features and have them updated.

3058 works for me. Could you share a screenshot of your features window? (FileFont Info…Features)

Hm, looks good to me. You could try the latest cutting edge release of Glyphs (select GlyphsPreferences…UpdatesShow cutting edge versions and click Check Now).

The culprit turned out to be a duplicate period, and Glyphs 3.0.3 (3086) could spot it.
Thank you!