The "index has wrong digest" logged error is probably the result of the download failing so you end up with a truncated .xml.gz file that doesn't match the published sha256 checksum. Add-ons are hosted directly (not mirrored like release images) but the addon server infrastructure is not overloaded and there is no active traffic management (bandwidth throttling) on our side. If there was a global problem with our infrastructure we'd see a ton of user reports, and we're not, which suggests a localised issue; something about your network or the connection between you and our infrastructure.
For kicks you can experiment with a current nightly on a spare SD card, but you should see the same problem since your requests are hitting the same server/infrastructure on our side.