@Hersfeld My procedure works!
The problem was the restored config. There is something in it that makes the sysupgrade think it is an old image. I did not go through the trouble finding out what it is since my config is so simple. I just performed a reset and configured by hand again! 
Yes, you do:
- backup your settings (just in case you wanna go back to 23.05.5)
- Use the sysupgrade upload in luci and upload the factory image and force the install (dont keep settings!)
- After the reboot its the factory image without overlay etc
- Use the sysupgrade upload again and install the sysupgrade image
- After the reboot configure your gs1900 manually again