What was going on
Uploads with the b2 CLI fail in the middle of the transfer with ERROR: More than one concurrent upload using the same auth token, killing transfers that had already uploaded tens of gigabytes. The CLI was reusing the same auth token across concurrent upload threads, which the B2 service rejects. Observed on CLI versions 4.0.1 and 4.1.0.
What fixed it
The fix is to upgrade the b2 CLI to v4.6.0 or later: maintainer olzhasar-reef identified the bad auth-token reuse and another reporter confirmed the upgrade resolved it. If you switched to another uploader as a workaround in the meantime, you can switch back after upgrading; the issue was closed after the reporter confirmed.