Comment (by Roland Haas):

I have seen GItHub dropping connections when too many concurrent connections existed. Eg this happened on our tutorial server when a whole class of students tried to download at the same time. Possibly a similar situation exists for you.

You may see success by using --no-parallel instead of --parallel (simply removing the option may not be enough).

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2535/et_2020_11-build-issue