I had the same problem on my new S20+ 5G. Also copied over all content from my previous S9. After a lot of tries I resetted the app preferences (Settings -> Apps -> Three dots -> Reset app preferences). Then going back to "Sounds and Vibration" I went on setting my ringtone the common way: Ringtone -> Sim1. I then was asked with which app I want to complete the action. Instead of my ES File Explorer I choose "SecSoundPicker" and voila, it worked again.
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