I did not solve it, so I made tests with Fieldset (Page). There also is a Repeaters section but with my fieldset.
It's not what I wanted but the Fieldsets are deleted when I delete a page manually or via api so that there's no waste with orphaned folders and files.
The stuff with the orphaned files and folders and entries in admin/Repeaters/ ... still exists and could not be detected with Ryans script (clean-files.php).
Could this then be a bug?
Thank you for your help
Carsten