[_Anima_]

I also use fastmail and I copied your config, with the only difference I'm saving in a local folder.

I'm getting this error for each email: IMAP error: unable to parse INTERNALDATE format

Do you also get that? Is this a problem?

I use this Docker image: https://github.com/JakeWharton/docker-mbsync

[spinax]

I am using Debian isync-1.3.0-2 and it's working - that Dockerfile shows it's using 1.3.3 and I spot a possible fix in 1.3.4 which could be related (or not, who knows). Your running mysync version might be different depending on the last time you rebuilt that running docker instance, the Dockerfile doesn't seem to pin it to a specific release version.

The Dockerfile is using Alpine Linux and the repos that it points to have already updated to isync-1.4.2 two days ago[2] - in general, I will make a guess your version of mbsync/isync has a bug? You're sort of using some random version, basically.

[1] https://sourceforge.net/p/isync/isync/ci/c8b73acad2a3c698d82...

[2] https://nl.alpinelinux.org/alpine/edge/community/x86_64/