Nagstamon 3.2 has a bug in its macOS version. Clicking the green icon in system tray (which way too seldomly appears) crashes the whole application. See https://github.com/HenriWahl/Nagstamon/issues/530.
This issue is fixed in the first testing version of this new year. Get it from https://nagstamon.ifw-dresden.de/download/#testing.
Edit: This fix now became stable release 3.2.1.