Most mod issues with Insurgency Sandstorm can be fixed by forcing the game to reauthenticate. To do this, follow these steps:
1. Close the game
2. Right-click on the "Start" button in Windows
3. Click "Run"
4. Enter %localappdata%\Insurgency\Saved\Config\WindowsClient\
3. Rename the file "GameUserSettings.ini" to "GameUserSettings.ini.backup"
4. Restart Insurgency Sandstorm
5. Log back into the mod menu
Then you can re-authenticate and it should work as normal.