Fatal: could not read Username for 'https://github.com'

It seems to be an issue with Github, found a discussion here: Unauthenticated clone of a public repo intermittently returns 401 (prompts for Username) from CI workers · community · Discussion #206581 · GitHub

Hey everyone! :waving_hand: Thank you for raising this. We have shared this with our internal teams to investigate - if we have any updates we will post here and in appropriate discussions :sparkles:

As possible workaround, but I would encourage you to wait first, this works:

cd /var/discourse
git config --global http.version HTTP/1.1
./launcher rebuild app

(you might need to rebuild two times because PostgreSQL upgrade)