I just discovered that for Regular Members (non-admin) that if I (as an Admin) use the Move Board function, that board appears just fine for me (as Admin), but as a Regular Member, that board vanishes from existence. The way to fix this is to Modify the Board and set the position using the drop down boxes. I do not have errors in my log.
@Senkusha - did you ever get help for this?
I have confirmed this problem
Bug report created: https://github.com/SimpleMachines/SMF/issues/7539
Marking solved then to indicate to other team members they can skip this topic if they wish, even though the problem itself isn't solved.