Here:
Ok first of all, in the console.log file, I saw this when Blockland was loading client-sided add-ons "Cannot use this add-on in demo mode". Don't you always have Blockland in retail mode? Why is it in demo mode?
The next thing is, when hosting servers do you have the Disable UPnP box checkmarked? Because this is what happend when you hosted a server:
Starting Internet Server
Binding server port to default IP
UDP initialized on port 28000
Initializing UPnP...
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UPnP Discovery Failed
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UPnP may not work for you, if you port forwarded Blockland to your computer, checkmark Disable UPnP from Options, Network.
Another thing that could be breaking your server is the RTB add-on. Maybe you should remove RTB from your add-on folder and see if that fixes the problem. Here is some broken RTB code that Blockland tries to load:
Add-Ons/System_ReturnToBlockland/modules/server/authentication.cs (0): Unable to find object: 'RTB_Networking' attempting to call function 'createFactory'
BackTrace: ->ServerSettingsGui::clickLaunchGame->createServer->onServerCreated