Here is the actual error.
NullReferenceException: Object reference not set to an instance of an object
at GameManager.PlayerId (System.Int32 _playerId, System.Int32 _teamNumber, PlayerDataFile _playerDataFile, System.Int32 _chunkViewDim) [0x0005d] in <4b1a8c809f0943a384283687c21422f8>:0
at NetPackagePlayerId.ProcessPackage (World _world, INetConnectionCallbacks _netConnectionCallback) [0x00000] in <4b1a8c809f0943a384283687c21422f8>:0
at ConnectionManager.ProcessPackages (INetConnection _connection, NetPackageDirection _disallowedDirection, ClientInfo _clientInfo) [0x000c4] in <4b1a8c809f0943a384283687c21422f8>:0
at ConnectionManager.Update () [0x00109] in <4b1a8c809f0943a384283687c21422f8>:0
(Filename: <4b1a8c809f0943a384283687c21422f8> Line: 0)
Looks like the player data file is corrupted. Find their files in the players folder (found by Steam-64 ID), back them up, and then delete them.