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Hi @davinicm, @bagarwa, @Insipidity
Sorry but this issue may be caused by various facts under different network environment. It's not easy to say for sure what is exactly the problem. The general suggestion we can give for people facing this issue is as follow,
1. A hard reset to your router and then make sure your device and desktop are connected to the same WiFi netwrok. Retry AirDroid.2. Check if any VPN or Firewall settings on your device or your router that may affect the connection.3. Ping the IP address generated by AirDroid on your device without the port number from your desktop to check if the connection is good.
4. Try another web browser or clean the cache and retry.5. Try AirDroid under another network or from another desktop to check if the problem is with your device or your network.6. Install WiFi File Explorer (https://market.android.com/details?id=com.dooblou.WiFiFileExplorer) and see if it also fails under same network.
May also try the methord provided by @Schrader. Good luck! :-)