When a server is initially joined and the scenery is generated, there appears to be a small window when other players can join. Once the server has players on it any other joins cause a crash. Here is the log from the join request to the crash.
This only appears to happen when running on a Linux based server suing Wine, such as a self hosted docker image or any of the purchasable servers such as Nitrado or IndefferentBroccoli.
When running a dedicated server on a PC that is also running a client there appears to be no issues.