I'm a noob to android, and I found this place via a Google search a few hours ago. I had just recently rooted my phone, so I have been doing all sorts of things.
(I'm about to go off-topic for a second...)
While looking through my Juice Defender Ultimate, I happened to see a message under the "Brightness" setting that warns that if it is turned on, there is a bug if you happen to install .apk files manually. By turning it off, I was finally able to install a theme on Metamorph (another issue I was having).
While I was trying to find a resolution to the Metamorph issue, I had been having the same "Sync to Dropbox" failures in Titanium Backup. I have just realized that ever since I disabled "Brightness" in Juice Defender (and thereby fixing Metamorph), my Sync to Dropbox isn't failing anymore (I had been closing and re-opening the sync all this time while surfing for answers in order to sync it piece by piece). It will fail if I push it to the background and start to multi-task, but as long as it is left in the foreground, it will continue syncing without failing.
Perhaps you guys probably don't have Juice Defender installed (or, if you do, maybe the Brightness setting isn't on?). If you do, maybe you guys can check to see if it works for you. If not, maybe there's a similar app causing issues too?
I know I will miss the Brightness setting, though. It seemed quite useful.