MP Non-OP players sometimes cant move, or break/place blocks after spawning

Server for occasional play for one or max 3 people, run from own computer. Everyone that has joined my server says they've experienced some part of this issue, but on my own account where I am OP, it never happens.
I still see them having the issue(s) ingame though.

The following happens with everyone except me, sometimes restarting the PC and/or server helps, but not always
-First, the user will spawn underground, then be ported around and eventually land on the surface. They will not be able to move, and their character doesnt move, even if they eat you only hear the sound, but no animation. When he tries to sprint you can see the particles at the feet, but he still doesn't move. When you remove the blocks beneath him he doesnt fall and gets kicked for floating.
From their perspective they move a bit, and then get glitched back to where they were before, like if they were attached to a rubber band. Thy cant break/place blocks or open/interact with doors/ovens etc.

This happened only once right ater a situation like described:
-All of the sudden the user can move, but he still can't look into chests/ovens etc, chests stay open after he clicks on them. While he can break and place blocks, he cant pick them up, or if he can it lags behind 30 secons even though the "tick" is less than 10ms and the block placiing/breaking is instaneous.

• While this happens, no errors appear in the console, but when closing the server right after, the attached crash-report was created.

Like I said, this happens every once in a while, and while it has disappeared before I cant reproduce or make it go away at will.