Appium Error: Cannot find any free port in range 8200..8299

I'm running about 90 tests in Appium (Android emulator on iMac) and it was all fine until suddenly I started observing this error:

     E       selenium.common.exceptions.WebDriverException: Message: An unknown server-side
 error occurred while processing the command. Original error: Cannot find any free port in range
 8200..8299}. Please set the available port number by providing the systemPort capability or
 double check the processes that are locking ports within this range and terminate these which
 are not needed anymore

I did a few things to fix this problem but nothing worked:

1.

adb kill-server    
adb reconnect

2. I did clean the emulator and restarted too.

3. Apart from this, I didn't find any port from 8200-8299 which is already being used in the system.

4. I did add systemPort capability as well but still I see the same error.

I have no idea how to fix this.