Same problem here. My client encountered a fatal error this morning after logging into wp-admin. Things I tried:
I manually updated the iThemes Security plugin via ftp so see if it was a stuck update problem, but the error persisted.
I renamed the plugin folder to disable it, and I was able to log in.
I updated WordPress and all other plugins prior to re-activating iThemes Security, and the error came back.
I turned on debug, but it reveals no other errors beyond this one.
The full error message is
Fatal error: Cannot declare class ITSEC_Schema, because the name is already in use in /path/to/site/wp-content/plugins/better-wp-security/core/lib/schema.php on line 3
WordPress 5.7.2
PHP 7.4.19
iThemes Security 7.9.1