Hi there, @DrMarcia.

I understand your how you feel, especially after being a loyal customer for over 25 years and investing in the Windows 11 upgrade as advised. Rest assured, I'm here to provide you the information you needed for you to get back in business as soon as possible.

The combination of older software and a newer operating system can lead to unexpected issues. QuickBooks Desktop Pro 2021 was only fully certified for up to Windows 10, and while it can run on Windows 11, it is possible to encounter problems, which could be the root of your current login trouble. More importantly, it's essential to be aware that QuickBooks Desktop 2021 was officially discontinued on May 31, 2024. This means you no longer receive critical security updates, and access to services like online banking, and payroll is discontinued.

As of now, we can troubleshoot your QBDT using QuickBooks Tool Hub. Check out this article for the detailed steps: Fix common problems and errors with the QuickBooks Desktop Tool Hub.

If the issue persists, I recommend rolling back to Windows 10 where you can run QBDT 2021 compatibly. Otherwise, you'll need to upgrade to a newer QBDT version to get access to all updates and services.

If you want to keep your Windows 11 operating system, you can ask your IT Tech to setup your computer to dual-boot Windows 10 and Windows 11. Every time you turn on your computer, you'll get to choose whether to booth Windows 11 or Windows 10. 

Please feel free to leave a reply if you have further concerns. We're always ready to assist.