Once world upload is 100% successful and it says "Verifying your world" the interface consistently mistakenly returns this error:

Upload failed! (500)

Yet this is consistently not true, the world actually being there having uploaded successfully.

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Also, when trying to initiate an upload to begin with, it takes many tries before it accepts (like trying to jam a wrinkly dollar into a vending machine). Today it was my 11th try that finally succeeded.

The error it returns on these aborted tries varies, but typically fails between 6% and 7% with:

Upload failed: (-1)

I've not had an upload fail that made it past 12%.