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Hey! I had the same problem! Both apps were up to date and I have tried to de- and reinstall both, without any changes. Then I had this idea to check, if I had a problem with my Spotify or Instagram account. So I signed out from Instagram and logged in a friend’s account while she logged in my account. In my theory, sharing songs on Ista story should now work for me and not for my friend. Contrary to my expectations, it worked on both accounts and when I signed in with my account again, it still was still working. Maybe this is working for you too 😊