See https://developer.apple.com/forums/thread/665666, fixed the issue for me. Specially changing to RSA SSH keys.

Try in Terminal: defaults delete com.apple.dt.xcode DVTSourceControlAccountDefaultsKey defaults delete com.apple.dt.xcode IDESourceControlKnownSSHHostsDefaultsKey sudo killall ssh-agent

Thank you GregVideoVillage. Unchecking "Upload debug symbols..." worked!! I mean it's abad fix, I like getting the crash reports already symbolicated, but at least I can get updates in the App Store for Big Sur. I had downloaded older version of Xcode and actually managed to fix the bug by using Xcode 11 command line tools in a script. But when uploading to the store from within Xcode it ignores the set version and uses the latest. Still present in Xcode 12.3. In a follow up, I have to restart Xcode between uploads as won't find a file on the second try. The file is always the quicklook generator and spotlight MD Importer. As I think it might be what was giving the original bug. It finds it once, but not twice and hence why the URL was empty and the crash that follows.