My first suggestion is to clear your cache.
In Internet Explorer, go to Tools >> Internet Options >> General >> Temporary Internet Files. Press the Delete Cookies button. Then press the Delete Files button.
Then close Internet Explorer and open a new browser window. Go to your site and test.
Do not log in prior to testing. If this works, then log in and test again.
If you get the same error in either one of your tests, this may be something that you want to report via the ticketing system.
As a new store owner, I know that there are things that you want to test, etc. I have placed items in my shopping cart, then gone back to my Admin Panel or Product Editor and made changes that affected that cart or the items in it. Often, I have gone back and forth like this several times, and eventually got an error that I couldn't get rid of.
These errors are completely understandable because changes to shopping carts normally happen on the front end, not the back end. Manipulating your cart or items affecting your cart on the back end may invariably affect the cart adversely on the front end.
The solution for me was to clear my cache, get back into my shopping cart, and clear the cart. Then I could start testing again with a fresh cart.
I don't know if this is the same type of problem that you are experiencing, but, when reading your post, it reminded of the problems I just described. Maybe this will help you.
Just my 2 cents.