Trouble with server

[FarmerExtroadinare]

My server is connected fine but barely any people can join it. People try to join and it starts sending files then it says this:

Add-Ons/TeamDMv4/GameMode_TeamDM_Base.cs (1712): Unable to find object: '' attempting to call function 'removeMember2'

And then they are disconnected. This happens to about 95% of the people who try to join my server. Does anyone know why and if so could you help me try to fix it?

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[Destroyer]

You probably have to forward your ports.

[FarmerExtroadinare]

i did, occasionally people can join my server. Most of them connect, then like 10 seconds later it says

Add-Ons/TeamDMv4/GameMode_TeamDM_Base.cs (1712): Unable to find object: '' attempting to call function 'removeMember2'

and kicks them off the server