Problems with FE authentication after TYPO3 upgrade

[cstrizzolo]

Hi everyone,
I am trying to upgrade my TYPO3 11.5.41 instance running ig_ldap_sso_auth 4.0.2 to TYPO3 12.4.24 with the same version of the extension, on the same host running PHP version 8.1.
My TYPO3 instance uses LDAP autentication for both FE and BE users (except the administrator one). Some users are allowed to log both onto the BE and the FE, while others only onto the FE.
After the upgrade, BE LDAP users can authenticate as before, while FE users can not. More precisely, the same user who should be allowed to log onto both can connect to the BE but not to the FE.
Apparently, no attempts to connect to the LDAP server are even performed during the FE users authentication process, according to the LDAP server log files.
I haven't changed anything in the configuration of the extension. I have also tried to uninstall the extension completely, reinstall it again and configure it from scratch but without success.
Another strange thing is that the LDAP / SSO icon in the left column is a black box instead of the correct ig_ldap_sso_auth icon.
I am really puzzled.
Any hints?
Thank you very much in advance,

Claudio