FOr the blank screen- i would have to say it means something is not setup correctly. HOwever, if your redirected to google and then in the GC you get the screen i think something is wrong with google. If you log back into your google account are the orders there? or are the no where to be found. after ordering on GC you should get a google screen which will advertise for google and have a redirect link back to your store.

I have no expierence with this screen but i have posted a lot of steps in the last week or two on this mod.

Milo- are you getting the error message in the ssandbox or production or both. Also, i went on your site tried to order like 50 of something and thenm google goes gray- google won't take orders on out of stock items. But hereis maybe a solution-
inflate your inventory numbers on everything that is low- then keep track of them to see when someone buys something you dont have- if oyu don't i figure your going to have people leaving because you are out of stock- this predicament sucks if you planned to let people order out of stock items and then just ship them when they came in.-