Do you mind me asking how you updated? The following warning should have been displayed for most updates (I don't think the linux update script shows this).
IMPORTANT: This release will ask you to upgrade your sync target. Once it is done, only apps version 2.3+ will be able to sync with it. So far the Android and Desktop app 2.3 are available as pre-releases, but iOS and CLI apps are not. So if you are frequently using these two apps, wait for them to be available before installing this release. See the release post for more information.
It does use the cryptic phrase "sync target", but otherwise this feels fairly clear that only updated applications will be able to sync.
It does appear that the android pre-release is not quite available which is regrettable. It should be available shortly.
Perhaps you could lend some of your considerable experience to this project for the next release?