In this post, I will show you how to fix admin login not working after upgrading to Magento 2.4.4 or above from older versions.
Magento 2.4.4 has an update on admin session size which causes this issue. In version prior to Magento 2.4.4, the default admin session size is set to 0; however, since 2.4.4, Magento 2 will perform a check on the size and disable admin session when the session size is set to 0.
To fix it, you need to have command-line access into the Magento project and execute the following command to set admin session size:
$ bin/magento config:set system/security/max_session_size_admin 2560000
After that, clear the cache with
bin/magento cache:clear and remove browser cookie.
Try to login into admin backend again. It should work at this point.