[signum]

I get the same error as you. I am not a big user of plugins, only once in a while. Checked a bit further and the one I just tried (it's very old; one of the first) is registered to use Python 2. Not sure that even exists on my system just yet. It's very simple so it might just work if I tell Sigil to use Python 3 instead. I'll try that later.

Just in case, can Python 2 and Python 3 co-exist simultaneously? How? Do I just install Python 2 with Muon? What new problems would I likely encounter?

Edit: Just re-read your post for anything I missed. It looks like you are using a new-ish plugin that is known to work with the default Python 3. Looks like a bug to me. No idea who to report it to or how.