Help please solve the problem with gamertag

I would advise that you
Contact Xbox Support for more information regarding this issue. You can always Request a callback or Request to chat with an agent.

please try removing your gamertag from the console and re downloading it using the new gamertag. make sure you remove profile only and not all your games and apps.

Is this with the xbox one games xbox 360 games All games?

Have you tried removing your profile and adding it back to the system?

For the sake of account security, I changed the account login and after that I inadvertently changed the gamertag to another one and now when I start the games I get an error.

What do you mean you "inadvertently changed the Gamertag"? Did you by accident create a new Microsoft Account instead of changing the sign in alias of your existing account? Because changing the Gamertag can only be done on a specific website or on the console
and will cost you the equivalent of about 10 US$.

As for account security: The only way to make your account secure is by enabling Two Step Verification on

https://account.live.com/Proofs/Manage.

Changing a login name is simply "Security through obscurity", and is not a method to make something secure.

Hello!!

Contact Xbox Support for more information regarding this issue.