My guess is an upload was corrupted, and is now ‘clogging’ the upload queue.
To fix this, the first step is to clear the upload queue. Are you currently logged in (seeing the main screen with Create a Hunt and Play a Hunt squares), or at the introduction screen? If the former, go to Info->Diagnostics, and tap Clear Cache. If the latter, hold two fingers down on the screen for about 8 seconds, then release. You should see see a notification saying the cache has been cleared.
After that is done, go to Info->Diagnostics->Backups of previously saved hunts. You should see copies of saved versions of your hunt, most recent at the top. Open the one you want, make a small change (like removing the ‘Copy’ from the title), and then save it. If it successfully saves, you should be navigated to the hunt overview screen of the hunt.
If it doesn’t successfully save, I’m guessing there’s a corrupted image in the hunt. Step through the different images, looking to see if any are blank (this can include hints and rewards).