LAN connection - blocks don't load - gamebreaking

Description:
I can join via LAN, but everything I can see there is a sky. And vice versa, I can see a player joined via LAN, but he's not moving and doesn't receive any damage. After a while, the player recevie "Connection Lost - Disconnected" message.

How to Reproduce:
Join a world via LAN.

Possible cause:
Different versions (1.14 v. 1.14 Pre-Release 5). Verification needed.

Solved, however the same issue appeared with identical versions later again.