Thank you for posting here in QuickBooks Community. I'll share troubleshooting steps that'll help fix some unusual behavior. Beforehand, may I ask if the user receives an 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 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 has the ability to access your QuickBooks account can't log on.
Use these keyboard shortcuts based on the browser:
For Firefox or Microsoft Edge, press Ctrl + Shift + P.
For Chrome, press Ctrl + Shift + N.
For Safari, hold down Command + Shift + N.
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 user 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.