New Post: the Cart is not empty, the order not cleared

Hello Rob,
thank you for your time and help.
I've spent all my morning testing , and this is what happened:

I used IE explorer, that I normally dont use, and it worked, both on cancellation and complete order placement.

Then I went back to the other browsers (firefox and chrome).
Before any order I purged NBstore, and restarted DNN, cleared browsers cache.
result: fail.

Then I installed Opera, never used it...
same result.

One thing that was interesting is that both firefox and opera, during the failing redirection to the site after paypal successful payment, pop up with warnings (this is the opera :" Information from this secure page will be submitted to a page that is not secure on www.xxxxxxx.xxx. Submitting sensitive information is strongly discouraged." - firefox has the same meaning), while IE explorer tells nothing about it.

Is it a security issue that is freezing the procedure in firefox and opera (because of more strict policies) and that is not in IE explorer (because of less conservative settings?)

p.s. I've tried also to redirect directly from paypal, same result.

What could it be?

thank you
have a nice day