Songs from my iTunes library with downloaded music file that are playable on my MacBook do not play on my iPhone.

Before I subscribed to Apple Music, I did have a library in iTunes with the files residing on my MacBook, that I synced to my iPhone. Since a few months I subscribed to Apple Music. Some of my songs, previously playable on my iPhone cannot be played anymore. I get a message that the content might be damaged. Sync library is switched on (yes I tried toggling this). I tried everything including a complete reset of my iPhone (signing out of iCloud and signing back in) but the problem remains.

Even when I switch off sync library on my iPhone, and connect it to my MacBook and sync the library I have on my MacBook, the song does not play on my iPhone. During sync I get errors ('unknown').

So the songs do play on my MacBook but not on my iPhone. What can I do?