I am experiencing the same issue for large world files on my android. Continually saying "there was an error" or just getting stuck on "initializing upload" screen after it already said upload complete.
I am able to play my large worlds offline just fine, but nothing seems to be able to upload. All of my worlds are over 50mb. It keeps saying there was an error.
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I tried uninstalling and reinstalling minecraft on my android. No avail. I am able to play the worlds offline. There are no mods involved. I'm worried my world may be gone from the realm forever.