Thank you for posting here in QuickBooks Community. May I ask if the admin user receive any error message when trying to log in to QuickBooks? I'd appreciate any further details so I can present accurate information.
In the meantime, I suggest advising the user with Admin access to log in to the QuickBooks account using a private browser. Doing that will verify if the issue is caused by a full cache. This could be also the reason that a user with Admin ability can't log on.
You can use either of these keyboard shortcuts:
For Google Chrome browser: Ctrl + Shift + N
For Mozilla Firefox browser: Ctrl + Shift + P
For Safari browser: Command + Option + P
If it works, then the main browser may have too much historical data from previously visited websites that cause this kind of behavior. That said, your admin needs to go back to the regular browser and clear the cache to delete those temporarily stored files and browsing history. They can also use other supported, up-to-date browsers as an alternative.