PI 3.1dev2 + uwsgi + nginx : connect to PI's unix socket FAILs (111:Connection refused)?

Welcome to the privacyIDEA community.
I am sorry, I did not read all your details. I am also not the nginx expert.
But you might look at the configs for our Ubuntu nginx setup here


which might give you a clue.

Check which distribution you are running on if it could be an selinux issue.