Projects problem with git credentials

I tried using the Node-RED Projects feature but can't get it working.

running as docker container:
image: nodered/node-red:latest
with a reverse proxy between, doing the authentication part (if that matters).

Node-RED version: v2.0.6
Node.js version: v14.17.6
git version: 2.24.4
"raw" installation, so no (additional) modules installed

Error message, when I try to create a new project:

I tried multiple git credentials but these are "not working":

also if I generate a ssh-key it didn't work.

And I can say, my docker image is up to date, as the output of the following command is the below:
docker image pull nodered/node-red:latest

latest: Pulling from nodered/node-red
Digest: sha256:ddd05759e91dce9db82d36a3df66011936cc49d59ac329589df379c44e74867e
Status: Image is up to date for nodered/node-red:latest

If you have any ideas or need some informations, don't hesitate to write to me.
Thank you.