Cannot add new glyphs - generic error

I’m trying to add glyphs to an existing font using Glyphs Mini version 1.0.3 (21). Every time I try, I get the following error:

The operation could not be completed.
No other information is available about the problem.

Is there a way I can determine the nature of this problem?

Additionally, I can add glyphs when working from a new / blank file.

can you duplicate the file in the Finder and try to add a glyph to the new file?