An update on this issue: I managed temporarily to have access to my files again. What I did was that I needed to downgrade to 1.5, create a dummy vault with 1.5, copy the vault.cryptomator and masterkey.cryptomator to the folder with my files, make a recovery with the initial plaintext recovery phrase to reset the masterkey. After upgrade to 1.15, I could open the vault again, but needed to update the vault version. I was able to access th access the files then.
However, some time later, I ran into the error that I cannot open any of my old files. It consistently gives me an I/O error in windows. I can however open new files which I created meanwhile.