Thanks. I am initially wondering if the song or playlist you are sending is corrupt. Do you have an exercise song you can test sending and make a playlist with a few of them in there, then send that playlist? (The exercise songs list under Composer, Exercise.) Or any songs from the Jazz 1300 playlist? (these should all work correctly).
In other words try some other songs or playlists and see if they import ok. Send from both your iPhone and iPad to your Mac, because one of them might be corrupting the song files but the other is ok.
And can you send the song from iPhone to iPad ok?

I assume you have tested import of songs and playlists from a browser (like Safari or Chrome) on your Mac to check they import? Does this work?

Also you could try a different email address (meaning based from a different ISP).
If you have Dropbox (or similar) you could try sending the html file there, then picking it up from you Mac. (You will need the Dropbox app on your iOS device.)

Thanks