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In the Phabricator comment I linked above, Niklas confirmed that this was a bug with a relay used by TWN for a short time (it was not the old one, but not the current one either), not with my software. By the way, I’ve been using the same, quite mature desktop application (Thunderbird) for years now, so it seemed very unlikely that it suddenly broke (and it was not even after a major version upgrade, but months later).