I've only been working with WordPress a few days and I had the same problem when testing my shopping cart. I couldn't go to the Shipping tab because the error wouldn't allow the page to load. The way I "fixed" it was I clicked on the shipping tab, copied the url and opened it in another browser. I logged in and it loaded the shipping page. Not sure why it worked but it did. I was then able to turn off UPS and that stopped the error. I also found next to the shipping choices of UPS, USPS and Australia that "edit" shows up when you mouse-over, when you click it the settings appear on the right and you can select "Use Testing Environment" or "Use Testing Server". Until you have all the set up complete. Not sure what will happen if I turn it back on now but at least I can finish setting up the site and I'll cross that bridge when I get to it. I only had 3 items in the system so far and was just messing with it so hopefully once all the shipping is set up and items are real I won't have any errors. So, that's what worked for me, hope it helps someone.