I found a general solution: Upload using Transporter app from Apple: https://apps.apple.com/no/app/transporter/id1450874784?l=nb&mt=12 Have to build using the Custom option, select "App Store Connect" then Export (this takes some time). You then get a PKG file to upload. It gives way more detailed messages around what is failing with the build on upload. My example: Can also open the menu to see more details processing info.

I reinstalled MacOS now (2021.04.25) with Big Sur 11.2.3, and the Python version is till 2.7.16. Check by running "python --version" in Terminal.