Minor I'm currently investigating failures with a ~16GB backup file. In our system, anything which pauses without displaying progress for more than about 5 minutes will cause a timeout. This also applies to Firefox users regardless of installation details; Firefox has a 5 minute timeout.
The step backup_zip_contents calls backup_general_helper::get_backup_information_from_mbz which takes over five minutes (with the tgz packer) and does not display progress.
The underlying function this calls does provide progress information so it should be easy to add progress reporting to this step and avoid the timeout. I hope!