I have an update to share on this. GitHub reported that they took measures to fix the problem:
I performed a test and found that the failure no longer occurs, even though the download took 12 minutes.
So I believe the problem is now truly resolved and the workaround I mentioned in my previous reply will no longer be required.
If anyone still encounters the "PROTOCOL_ERROR; received from peer" error when installing an Arduino boards platform, please comment here (or in the GitHub issue if you prefer) and we'll investigate further.