The android app just started throwing an error saying upgrade to version 3.7.0+ to sync. But the published version in play store and fdroid is 3.6.21. Was this mistakenly merged in from the dev branch maybe? And why would you break sync instead of making it backwards compatible? That could mess up someone with a selfhosted solution that can't conveniently update.
Sync from linux desktop app seems to be working.
Edit: at least one of the devs seems to be aware of the issue. I guess it isn't a quick fix, so they are suggesting android users manually install the pre-release package to fix this bug in the meantime. https://github.com/laurent22/joplin/issues/15753
I had the reverse situation with the joplin sync issue (PC outdated, phone newer version) and solved it by using a pre-release from the joplin github repo.
I am not happy about the situation that breaking changes are introduced without warnings and the versions of the clients aren't released on the same time.