"Problem parsing repository source" when opening Plugin Manager

I’m getting an error “Problem parsing repository source” on opening Plugin Manager. This is an error message in the Console.app:

2017-01-25 13:58:19.257 Glyphs[38518]: Error while downloading packages! Error Domain=Problem parsing repository source Code=123 "(null)"`
  • Glyphs 2.4 (939)
  • OS X 10.11.6

I fixed it.

Now it works, thank you!