I also downloaded a world I was playing on realms. I played it for awhile without adding to the map and then tried reuploading. I even downloaded my old realm world without any edits and tried to upload it 5 minutes later and it times out.
I am trying to upload the videos but theyre 406mb each so too large for upload on this site. My 8 year old son discovered the issue. It resets the world border size and location during the error. Once I changed the size to larger than where I was it worked normal in that area. I will take screen shots of both areas to let you know and hopefully make the video smaller.
Thank you. Realms is a great way for my kids and I to play on the same world we have created together.