Ok, back already.
I read the tutorial and remembered that I had seen on Github that there is also a server editon as appimage.
To test if there are problems with appimages, I tested those first. Downloaded and ran it, then ran it in the browser 127.0.0.1:8080. It works, my last backup applied and worked as well.
This does not solve the client problem but I could already enter some things and resave.
How long can I use the server version if the client is not working for a while?
systemctl status
─app-\x2fhome\x2fmarika\x2fDownloads\x2fManagerServer\x2dx64.AppImage-e6e897a44b28434ea77fe93858cd28e5.scope
│ │ │ │ ├─11784 /usr/lib/appimagelauncher/binfmt-bypass /home/marika/Downloads/ManagerServer-x64.AppImage
│ │ │ │ ├─11795 /bin/sh /tmp/.mount_Manage5KfP8t/AppRun
│ │ │ │ ├─11799 /home/marika/Downloads/ManagerServer-x64.AppImage
│ │ │ │ └─11803 /tmp/.mount_Manage5KfP8t/opt/manager/ManagerServer