This error is produced when a server name has been typed in the Assemble add-in server field more than once.
To resolve this issue:
1. Navigate to C:\%localappdata%\Assemble\ from Windows Explorer.
2. Delete the Connections.xml file.
3. Publish your model, but you will need to re-enter your server name.
4. In future publishes, be sure to use the server name from your drop-down list in lieu of re-typing the server name.