I don't have a log from that incident, but I do have to say, only with the NPC mod do I get this error, and this is also causing issues currently on my server..
Used the same mods in A17 and got the same error. We actually had to remove the NPC mod because of it. Wandering trader and Bandits mod works just fine.
Removed this mod, error gone. Put the mod back. turn on the server, error is back.
EDIT: Server is having the same issue we had back in A17.. players can't join after a certain amount of time, and our server hangs saying a player is still there, when they are not. Happens when they get around NPCs. A player said they hired an NPC for bloodmoon, and then the server crashed and kicked him off, but the server was still on, and unable to have players connect.
Also, this spam in log makes it extremely frustrating to debug any errors we may have... 15min of my server being on and my log file size is 12,623,245 bytes......