Hi @Heiner junker
Welcome to the Sonos Community!
Sorry to hear of this issue you are having with moving your system to another account.
If you are signed-in to the Sonos app (with an account that is not the account to which the system is registered), then you will need to sign out of the app before you can perform Transfer System Ownership (assuming that is what you are trying to do?). Go to the User icon in the top right, then tap Sign Out, before going back to Transfer System Ownership.
If, however, you are trying to move just one speaker from one existing system to another existing system, you will need to factory reset the speaker then set it up on the second system as if it were new (there will be a green flashing light after the reset).
I hope this helps.
