"CRC doesn't match, file is corrupted. It may be a temporary problem, please retry later"

I'm seeing the error message "CRC doesn't match. It may be a temporary problem, please retry later" when trying to use the Board Manager to install esp8266 from http://arduino.esp8266.com/stable/package_esp8266com_index.json. Sometimes there is no error message, but the the arduino IDE just freezes, and has to be stopped externally.
I'm not behind a proxy. I've uninstalled and reinstalled the IDE, pasted the .zip file into the /Arduino15/staging/packages folder directly, and deleted the staging folder. Nothing seems to be working.

No source file, test, or entry point is identified in the report. Start by reproducing Board Manager installation with the esp8266 package index and compare the CRC failure and freeze behavior, then identify the responsible component and add a regression test if the cause is local to this repository; done means the installation completes or the failure is reported reliably.

Written by the indexing model from the issue text.