Quoting Andreas Ronnquist <mailinglists@gusnan.se>:
I've got a new version of gajim in Stretch backports today, and after this, it reports this:
Yes, my bad! I'll upload the latest testing version of python-nbxmpp today to bpo. With that version it will work again. On my own machine I had an adhoc backport running and forgot about it :( Will be fixed in a few hours!