One thing to check would be your Server Options > Max BBS Nodes
Thank you. I have it set to 3, which seems like enough, but I will try increasing it.
Ok next step I would suggest is restarting your computer (this is of course optional, but sometimes turning it off and on again helps!)
But then with the BBS offline carefully delete "chat*.dat" from your DATA folder. Sometimes that data can get stuck there if there is a crash or a shutdown for some reason. For example, since I moved my BBS to my laptop I sometimes shutdown or crash my OS (thanks to latest Win 10 update) which will sometimes leave an old chat*.dat file that needs to be removed.
Basically that file flags a node as in-use.
--- Mystic BBS v1.12 A46 2020/03/22 (Windows/64)
* Origin: Sector 7 (1:129/215)