I’ve been experiencing the same issue with Duplicati backing up to a B2 bucket in Backblaze’s East Coast datacenter (list, upload, and delete work; download fails). I was able to isolate the issue from Duplicati by confirming that also I couldn’t download files using Backblaze’s b2 command-line tool.
This was the same answer I got from Backblaze after opening a ticket with them. I’ve since forwarded my complaint to my ISP, and I’d recommend you do the same (the more people complain about this, the more likely it’ll get fixed promptly).