Philoludos disconnecting: 
Object reference not set to an instance of an object.   
at ConstructionSkin.SpawnGibs (BuildingBlock parent) [0x00000] in <00000000000000000000000000000000>:0     
at BuildingBlock.SpawnGibs () [0x00000] in <00000000000000000000000000000000>:0     
at Client.OnEntityDestroy (Network.Message msg) [0x00000] in <00000000000000000000000000000000>:0     
at Client.OnNetworkMessage (Network.Message packet) [0x00000] in <00000000000000000000000000000000>:0     
at Facepunch.Network.Raknet.Client.HandleMessage () [0x00000] in <00000000000000000000000000000000>:0     
at Facepunch.Network.Raknet.Client.Cycle () [0x00000] in <00000000000000000000000000000000>:0     
at Client.Update () [0x00000] in <00000000000000000000000000000000>:0

Any idea what's causing it? Every time I undo, it kicks me from the server. 

Try to disable gibs in your settings.

Oddly enough, a server restart (for unrelated reasons) fixed it. Haven't had a problem since. 🤷‍♂️