maybe this could help others with the same issue. I finally discovered why importing from a Splash ID exported CSV file fails in Enpass 6.x: The exported file has old-style Macintosh line endings (CR), but Enpass expects CRLF. If you try to directly import the generated file in Enpass, the dreaded "nothing to import" error occurs.
To avoid this, open the CSV file in a text editor (such as BBedit), change the line ending style from "Legacy MacOS (CR)" to "Windows (CRLF)" and save the file. Now, the data can be imported to Enpass 6.x