Hi Jan. Thanks for your question.
We definitely want all our users to be on the latest version, which is 4.0, as you know.
Since you’re trying to upgrade from 3.12.2 (and the prior latest version was only 3.12.6), an automatic or manual update from 3.12.2 to 4.0 should work just fine. Of course, many of our users have updated to 4.0 already, from a variety of prior versions.
If the update to 4.0 isn’t going smoothly for you, I’d suggest deactivating all non-Modern Tribe plugins, activating the Twenty Fifteen theme, and then trying to complete the automatic update to v4.0 of our plugins.
I hope this method of updating helps you resolve your issue. Please let me know how it goes for you. Thanks!