So an update. It turns out that all the issues were actually to do with Postiz rather than Authentic. - I tried installing a few other apps and linking them to Authentik but I ran into a few issues. Then I took Postiz and tried turning off the Atuthentik authentication and then just getting it to work with native user accounts. This initially didn't work - it turned out that there were a few issues.

1. The startup procedure for Poztiz actually left the backend API service not running properly. I had to run the docker compose, get all the other services running and then stop the postiz container specifically wait a bit more and then restart just that container. That got the API service running.

2. Because I was just running it internally I didn't have SSL certificates configured and was just using http - consequently I had to introduce NOT_SECURED: "true" into the environment section of the Postiz service in the compose file and then repeat the process above. This resolved a silent error in my browser where cookies used to manage the authenticated browser session were discarded because I wasn't using SSL.

So success at least getting Postiz running. I am now going to step back, reconfigure it for Authentic retry.

I am going to guess that these issues might be relevant if you are running Postiz behind a reverse proxy (I'm not sure if this is being done with the docker compose set-up). I possibly also need to look at how much memory and CPU I have on my test VM.

I've posted similar details in Postiz Discord. If anyone else is using #Postiz and would like to exchange notes - please do drop me a mention. Always good to have other people to talk to about these things and share notes.