[Bug...probably] Map/ Entity interfering with Game/ Gamemode

The map that shows the problem is gm_Lair.
After running fine for some time, all dynamic entities (including the player) seem to be removed and the console is spammed with the following code (got the “bad coder” achievement in 10 seconds flat):

[gamemodes\sandbox\entities\entities\base_gmodentity.lua:22] attempt to call method 'GetEyeTrace' (a nil value)
BeingLookedAtByLocalPlayer	gamemodes\sandbox\entities\entities\base_gmodentity.lua:22 
unknown	gamemodes\sandbox\entities\entities\base_gmodentity.lua:33 D

Something in the map seems to be interfering with the gamemode or Gmod itself.
I’m trying to fix this this problem, which seems to be a part of the map, since other users have the same problem and other maps run just fine for me.
I just have no idea where to start looking for the problem though. I’m not a coder and Google doesn’t seem to know anything either, so any help or interpretation of the code would be really appreciated.