Upload stuck on "Preparing your world"

I downloaded a backup of my realm, added a datapack compatible with 1.13, and reuploaded the world. Upon reuploading the world, I'm shown a blank screen with text displaying "Preparing your world". The only option is to click "cancel" which cancels the world upload.

 I did find a single duplicate bug report, but it has remained untouched since its original report date.