Error prevents upload of local world to Realms in Minecraft PE

Just signed up for Realms and was excited to upload the local world I've been building in Minecraft PE on iPad. Imagine my disappointment when it consistently fails.

After I click "Replace World" and select my local world, it first seems to work - an upload starts - but then a pop up informs me "Error has occurred" and that's the end of that. Tried different local worlds, restarted the app, etc - no change. 

Grateful for quick help on this since if this is not resolved, I will be cancelling my subscription before the free month ends.