You may have to configure your firewall to allow AirServer to bypass the MacOS built-in firewall. In order to achieve this, you should add a firewall exception for AirServer for Mac within your firewall settings. 


Instructions:
  1. Open up System Preferences.
  2. Go to Security & Privacy.
  3. Select the Firewall tab.
  4. Click the lock if you are unable to press any buttons in the Firewall tab.
  5. Press Firewall Options...
  6. Press the + button and add AirServer.
  7. Make sure it says "Allow incoming connections" behind AirServer in the list.
  8. Make sure "Block all incoming connections" at the top is not ticked.
  9. Press OK.