After closely following the instructions given here:
https://support.atlassian.com/bitbucket-cloud/docs/deploy-build-artifacts-to-bitbucket-downloads/
and following step 3b from these instructions, we are unable to upload the build artifacts. It looks like an authentication issue.
We spent about a day trying to get this up and running. Also by looking at the community support questions and other documentation, but no worky. We have been trying to contact support, but this seems impossible via the website.