Actually, this problem happened to me a few years ago while using QT, so I can help clarify.
It's NOT caused by logging in to ToS software. I do that every day with no problem.
The specific problem is:
When you are logged in to the TD Ameritrade website, if you go to "My Profile" / "Elections & routing" / "Advanced Features *NEW*", and you see that you are "Eligible", DO NOT click on "Upgrade"!
Once you are "Upgraded", QT will no longer work.
If you have already "Upgraded" (as I had done), you can call TDA Support to reverse the "upgrade".