SOLVED: Jellyfin Server login works only via remote proxy but media details page never works

[DavidM42]

I recently moved my Jellyfin installation on TrueNAS Scale from TrueCharts to TrueNas Apps and copied all the configuration. I am using a caddy2 remote proxy to access it via https externally.

Even before the move to TrueNAS Apps (which I hoped would fix it) clicking on episode lists of shows or details of movies started to fail suddenly. In the browser, I see errors like 

> itemDetails.21d720f417042985d57e.chunk.js:2 Uncaught TypeError: Cannot read properties of undefined (reading 'getItem')
and
> Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'appName')

while trying to open the details pages.

This is the behavior while routing through my caddy2 remote proxy from outside. If I try to open it directly internally via http://192.168.178.IP:8096 I only get the login page which shows an endless blue spinner and following browser console error
> Uncaught Error: Server not found: 97eda71247654f86b30c89f31b53d8c3

I've included my jellyfin log in verbose mode, but I sadly could not find any problems or errors in it that helped me
https://gist.github.com/DavidM42/2dde8c5...dc0a850672

Thanks in advance for any help