I know I've seen a lot of people posting about this lately. I have created a few dozen worlds to test, both co-op and solo. I have noticed if I do a fully normal, default, balanced world gen - traders work just fine. If I do a full snow biome, or full desert, or 'some' variations of the biomes, the 'no trader' error pops up. It's not a name change issue as one person mentioned previously, as with a purely default gen world using this mod, it works just fine. Changing the world/biome setting doesn't affect names or prefab names. So, there is something else going on. I have yet to figure out what or why biome changes would affect initial traders - except (possibly) perhaps some traders are now coded to only spawn in specific biomes and it's causing havoc with the mod....?
Edit: just for clarification purposes, I have completely wiped everything from my previous patches and saves. Wiped every file and folder, started 100% fresh. It still happens on my rig and my wife's, unless it's a 'default biome' map.