I had the same problem earlier this evening. Unfortunately I was unaware of your "back end" problems. I finally switched to offline mode as a last attempt to get some work done and I was able to open my design, save changes and when I switched back online the hung upload cleared.
While I realize things are going to go "tilt" from time to time with any system, "back end" problems should not create local problems for your users. It would be nice have some of these work arounds automated so they happen in a manner that does not negatively impact the user. Fusion, or A360, could easily detect a hung upload condition, notify the user, flush the failed upload and open the latest version from the users local hard drive. This would require a local copy to be made of everything that is uploaded when its uploaded. If you scan through the blog you will find many users expressing concern over exclusive cloud storage because of worry about "back end" issues that can happen at any time, restricting access to their designs. I like the cloud, but I like the cloud with a local mirror more.