Experienced a crash in multiplayer (open to local LAN, not a stand alone server) with 1.15.9. Two players. Created a new world and played for a few hours and suddenly hit a crash. I was subsequently unable to re-open the world after this. New world creation and load is successful.
Probably unrelated, but this was within a few minutes of first pickaxe use and also the first time I noticed fog.
No mods active, though CarryCapacity 0.6.3 was installed and disabled the whole time.
The crash report below is when trying to re-open the world.