Troubleshoot “connection to KMS server failed” error

I have received a lot of complaint emails saying that they could not activate their program after folowing instructions in my videos. The issues described here are related to an error message they got as below.

The connection to my KMS server failed! Trying to connect to another one… Please wait…
Sorry! Your version is not supported.

There are two main causes of this issue.

1. The cause of Client
First of all, please make sure that Windows Update is enabled and your internet connection is stable. To check if my KMS servers are blocked or not, please open your internet browser then try going to: http://progwithsaurav.blogspot.com. If the page is visible, it means this server is not blocked.
One other point to note is that you need to run command prompt/batch file with admin rights.

Make sure you run as Administrator

Otherwise, the connection will be denied by UAC system.

2. The cause of server
Recently I need to do some experiments in the kms7 server so I changed the default KMS port (1688) to another one. And besides, I upgraded CPU and RAM memory on the kms8 server.
Because the number of daily clients is too big and still increasing over time, so the server can also be stuck at times.
To check if the port 1688 is open, you can use the telnet command on windows or use this online port check tool in simpler way.