multisite – Error Message when logging into WordPress on Xampp
Does the ‘bad’ site public-facing page display? If not, then you have database issues (doesn’t exist, permissions, user account, etc). Check the wp-config.php file to make sure that the database name/user/password is correct.
If the public-facing page displays, then use phpMyAdmin (or similar) tool to look at the wp-users table in the database specified by the site’s wp-config.file. Make sure the name/email is correct and the same as what you are trying to use. There are many googles/bings/ducks on how to change the password directly in the database.
Also, look at the error.log file for the ‘bad’ site to see if there are hints there as to the issue.