Hello @mzimmermann,
Have you looked at this Fedora Mag article about dracut? It may help with your boot problems, and provide info on how to get back up and running.
And from my own exeprience with trying to get to previous versions of Silverblue… this topic
Solved by @dustymabe with the following.
Replace the version of Fedora with the one you are running (31?) then use the ostree pull command.