Heavens! I finally managed, but I’m not sure through what miracle. I logged out again. Asked to connect, and hit cancel on the screen where it asked to use my instance domain to connect. Got the login screen with various options like DiscourseID, and this time typed in my username and asked for an e-mail link. The link arrived, I clicked on it and was signed in but… in the browser, not in the app. Didn’t find a way to switch from the browser to the app. So I went back to the app login screen, which was stuck spinning (probably waiting in vain for me to arrive through the e-mail login link) – I ended up having to kill it. Opened it again, opened my community, got the usual pop-up asking for permission to connect. Cancelled, was sent back to the screen listing my communities. Tried again, finally hit “connect” as “cancel” wasn’t working, and lo and behold, it actually logged me on with the right account!
But I’m baffled. Not sure how/why it finally worked.