Users cannot log in to new site in multisite
This issue might be a variation of the question about "drupal /?q=user is working but /user not working". So verify if you can use an URL containing ?q=user or ?q=admin. If that gets you going, then probably tuning RewriteBase in your .htaccess (as described in the accepted answer of that question) should help.

It might be worth trying to further enhance your .htaccess file, by adding a line after that existing RewriteBase, which looks like so:

RewriteRule . index.php [L]


For more details on the above line, refer to the (accepted) answer in this question on StackOverflow.

Another option for debugging is to retry using another browser. If via that browser you are able to login, then it's probably a cookie problem (in your original browser).