I finally figured it out. It was a DNS issue 
For whatever reason my router resolved the backblaze download hostname to an IP address that is no longer in use.
I have now switched over to 8.8.8.8 as my DNS server and everything works fine. I expect this invalid value disappears from my router’s cache at some point. Though dig shows a TTL of 0 for the entry, so it should have been removed already.