3000015 created the issue

When using the AWS-compatible backblaze S3 endpoint as in their docs, it seems that small files upload fine and fast, but large files stall. The upload never starts, there's a message about "initializing large file upload" but then nothing happens anymore.
The download can't be stopped or removed, only quitting Cyberduck brings it back into a working state (file upload still fails).

Cyberduck is used as example integration app in the backblaze docs:
https://help.backblaze.com/hc/en-us/articles/360047425453-Getting-Started-with-the-S3-Compatible-API

I'm not sure if that's an issue with CyberDuck, Backblaze, or their S3 integration, but since Cyberduck goes into a broken / unstoppable state there's at least something wrong here.