I have had this crop up various times and I can't find a strong correlation as to what event is causing it. I think it might be a conflict with the Rampant mod.

--------- Error from server ------------
260978.664 Error MainLoop.cpp:1285: Exception at tick 6301281: The mod Pollution is the Solution (1.0.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Pollution_is_the_Solution::on_entity_spawned (ID 133)
Entity is not unit.
stack traceback:
[C]: in function 'index'
__Pollution_is_the_Solution/control.lua:6: in function <Pollution_is_the_Solution/control.lua:1>
260978.664 Error ServerMultiplayerManager.cpp:91: MultiplayerManager failed: "The mod Pollution is the Solution (1.0.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Pollution_is_the_Solution::on_entity_spawned (ID 133)
Entity is not unit.
stack traceback:
[C]: in function 'index'
__Pollution_is_the_Solution/control.lua:6: in function <Pollution_is_the_Solution/control.lua:1>"
260978.665 Info ServerMultiplayerManager.cpp:780: updateTick(6301281) changing state from(InGame) to(Failed)
260978.665 Quitting: multiplayer error.

------- Mod List --------

{
"mods":
[

{
  "name": "base",
  "enabled": true
},

{
  "name": "ArmouredBiters",
  "enabled": true
},

{
  "name": "Artillery_Manual_Only",
  "enabled": true
},

{
  "name": "Big-Monsters",
  "enabled": true
},

{
  "name": "bigger-artillery",
  "enabled": true
},

{
  "name": "bobassembly",
  "enabled": true
},

{
  "name": "bobelectronics",
  "enabled": false
},

{
  "name": "bobenemies",
  "enabled": true
},

{
  "name": "bobequipment",
  "enabled": true
},

{
  "name": "bobgreenhouse",
  "enabled": true
},

{
  "name": "bobinserters",
  "enabled": true
},

{
  "name": "boblibrary",
  "enabled": true
},

{
  "name": "boblogistics",
  "enabled": true
},

{
  "name": "bobmining",
  "enabled": true
},

{
  "name": "bobmodules",
  "enabled": false
},

{
  "name": "bobores",
  "enabled": false
},

{
  "name": "bobplates",
  "enabled": false
},

{
  "name": "bobrevamp",
  "enabled": false
},

{
  "name": "bobtech",
  "enabled": false
},

{
  "name": "bobvehicleequipment",
  "enabled": true
},

{
  "name": "bobwarfare",
  "enabled": true
},

{
  "name": "FNEI",
  "enabled": true
},

{
  "name": "Kux-OrbitalIonCannon",
  "enabled": true
},

{
  "name": "Pollution_is_the_Solution",
  "enabled": true
},

{
  "name": "Portable-Turret",
  "enabled": false
},

{
  "name": "Rampant",
  "enabled": true
},

{
  "name": "RampantArsenal",
  "enabled": true
},

{
  "name": "rampart_heavy_wall",
  "enabled": true
},

{
  "name": "SchallArtillery",
  "enabled": true
},

{
  "name": "SchallMissileCommand",
  "enabled": true
},

{
  "name": "SchallTankPlatoon",
  "enabled": true
},

{
  "name": "Squeak Through",
  "enabled": true
},

{
  "name": "stdlib",
  "enabled": true
},

{
  "name": "what-is-it-really-used-for",
  "enabled": true
}

]
}