Just found this...
Posted January 10, 2016 ยท Report post
On Steam-enabled servers, mods are no longer installed the old way: you include the workshop ID and mod loading ID in servertest.ini, then the mods are downloaded and automatically installed for both the client and the server. These mods are automatically updated by Steam when the client connects to the server, or when the server is started.
If you attempt to install these mods in Zomboid\mods\ or ProjectZomboid\mods\, they will not work, or people may end up with multiple versions of the same mod.