If you are seeing the error message Invalid Auth Key on a Windows server, you can follow the below procedure to resolve the issue.
Go to Settings > Proxy Configuration
Click on Add a Proxy
Copy the key
Connect to the Windows server where the proxy is installed via RDP
Open a text editor (e.g. Notepad or Notepad++ as Administrator)
Open the file C;/Program Files/Faddom/Tomcat/Conf/faddom-proxy.properties (you may need to change file type to All Files)
In the line authentication key = paste the key after the = symbol.