SOLVED: Fresh Setup with Container on Synology

[Generaldirektor]

Hi all, 

hope someone has an additional idea. 

Problem: I am trying to create a fresh setup for Jellyfin using the Container Manager on Synology (which is actually Docker under the Hood, despite the new name) 
When accessing the Jellyfin page I see that it offers to connect to the server but it is somehow an old entry since it wont connect. Adding a "new" Server connection works, but it requests a login (not a new registration) and although I have the user's credentials I used on my old setup, the login fails. 

 I already tried to stop and delete the container and also the image. Re-downloading the image and creating a new container shows the same result. 
I then used the CLI (SSH on Terminal to Synology and switching to Root access and Sudo Shell) to dive deep into the docker folder. 
Using

Code:

docker volume prune

to remove old subvolumes from previous containers removed a couple which had jellyfin references in it. But even when removing all of them, and then setting up a new container still shows the "old" server and asks for credentials where the credentials used last time, do not work anymore. 

Where else can I find the remains of the previous installation to reset everything to a new start?
Or can I otherwise force the web access to show the initial setup wizard?

Best regards, 

Generaldirektor