Welcome to Adobe Forums.
Just want to confirm that you have SP1 installed for your Windows 7.
Also make sure you have SP1 installer for MS Office (http://www.microsoft.com/en-in/download/details.aspx?id=26622).
Does this Error comes on a blank presentation as well ?
As a testing try using Adobe Presenter 8 on a different user account on your computer.
Yes, I have SP1 installed for both Windows 7 and for MS Office.
I just now created a required-passing quiz with a blank presentation, saved it, reopened it and I was able to edit the quiz. I have tried several times, however, when I attempt to edit a required-passing quiz that was created in a course developed with Presenter 7. I have worked with Presenter 7 courses for 5 years and have never experienced this problem; however, I do frequently need to go back to these courses and make edits. Since installing Presenter 8, it has been very frustrating as I've had to delete the quizzes and recreate them.
I'm not certain what you want me to try as far as using a different user account on my computer (or how to do this).
Create a new user account on your Windows 7 (Start -> Control Panel -> User Accounts -> Manager User account)
Make sure you have Presenter 8.0.1 updates installed.