I am having the same issue with my friend joining a server hosted by me.
The console.log of the server states:
Posting to master server
Posting to rtb server
Got Connect challenge Request from (My IP):36688
Starting arranged connection to (Friend's IP):36688...
o possible[0] = (Friend's IP):36688
o possible[1] = (LAN IP that has nothing to do with ANYTHING, isn't my computer):36688
o possible[2] = (Unrelated IP, has nothing to do with anything):36688
o possible[3] = 127.0.0.1:36688
Got connect request from (My IP):36688
net name = EJBeierle
AUTHCHECK: EJBeierle = internet client -> internet server, regular auth
Authentication Failed for EJBeierle (My IP).
Issuing Disconnect packet.
I have uPnP off in both Blockland and my router. Port 28000 is forwarded to my computer. Attempts to join were made in Steam, with the server list, with my IP, and requests over RTB. This was done in both Steam and non-Steam versions of Blockland. I have tried this same issue with only one default add-on enabled (Emote_Alarm, picked randomly), and it still disconnected him.