Can you see if it acts the same in a local, solo game, with the mods disabled? That would give us a clue as to source of problem.
Not asking you to change anything on the server, just your local machine, temporarily. Re-enable them afterwards, one by one, and see which one gives that particular error, if you’re good with them disabled.
So I went through and added each mod one by one. It’s Less Building Placement Restrictions. I’ll contact the mod owner. Thanks for your suggestion! It seems like a really “duh” moment, but I’ve been up a long time trying to get my server underway, so my brain isn’t quite awake, lol.