Hello @Shuvajit Roy Thank you for contacting us. I understand you are trying to perform discovery of your Hyper-V VMs to migrate them to Azure. However, you're being greeted with server credentials error.
Since, you have experience working with the same environment and the process went through smoothly. However, this time it seems to be blocking your progress, may be due an intermittent issue.
Suggest you to try re-validating once again and see if that works. If not, please try to follow the guidance from the links below, just to ensure you haven't missed anything during the set up process.
https://learn.microsoft.com/en-us/azure/migrate/tutorial-discover-hyper-v#provide-server-credentials
https://learn.microsoft.com/en-us/azure/migrate/add-server-credentials
Hope this helps!