TYPO3 after upgrade from 10 to 11 the login not possible

First, it should be verified whether the selection in the Install Tool under "Settings / Configuration Presets / Password hashing settings" is set to green. This indicates that the password hashing settings are configured correctly. It's advisable to check other projects that might be hosted with the same provider to see what settings are used there, ensuring a consistent and secure configuration.

It may be necessary to switch from Argon2i to, for example, bcrypt. This change might be required to resolve compatibility issues or enhance the security of password hashing. Bcrypt is a proven method for securing passwords and offers a good balance between security and performance.

After that, ensure that the database is clean under “Analyze Database Structure.” This means there should be no inconsistencies or errors in the database structure that could affect the application's functionality. A clean database structure is crucial for the smooth operation of the application.

Then, the cache should be cleared. Clearing the cache ensures that no outdated or faulty data is used, which is especially important after making changes to the configuration or database structure. By clearing the cache, you ensure that all changes take effect immediately.

Subsequently, a new backend user can be created if the login still does not work. Creating a new user account can help determine if the issue is related to the existing user account. If the login works with the new user, it could indicate that the old user account is corrupted or has incorrect settings.

In summary, it is important to proceed systematically and check both the configuration settings and the database and cache to ensure that all components are functioning correctly.