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.
punnerud
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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.