voilà my problem:
upgrading from Debian Jessie to Debian Stretch and then from OMV3 to OMV4 worked pretty well on my RasPi2.
I am still able to connect to the installation via ssh an can access the storage via ssh, too. But the web-surface doesn't work anymore: after filling in the local IP address I get the Login-page, fill in name and password, then the screen gets a little darker, it says "Bitte warten...", the Address is shown in the status bar at the bottom of the page but finally there appears only the blue openmediavault logo in the upper left corner, the rest of the page remains light grey.
When I then clear the browser cache, there appears the red and black warning frame about a broken connection, and I can restart the same thing all over.
I think there might be a problem with the .json files in /var/cache/openmediavault, because after the upgrade, I had to restore them from a backup that I made before starting the upgrade.
Is it possible to reinstall only these files without losing all of the configuration?
Thank you for your help!