Just got my own Marshmallow update. SD card was recognised by the phone, but not by Spotify – "Storage area not found" and crash. Deleting cache did nothing. I had to delete all the data, cache, app, reinstall and now redownloading all the songs. It's one time though that I don't blame Spotify for the error, Marshmallow completely revamps the way SD cards are treated.
But: isn't it the responibility of the Spotify app to follow the rules? Android makes the rules, Spotify follows...???
If the data (ie. offline songs) is "stored/coded" in folders the exact same way from version to version in regards to Spotify's "format" or structure, and it's just the permission handling that is different, I would have thought an update to Spotify was the answer and not Android?