iTunes 12.2 - grey squares instead of play buttons

I have a strange little issue with iTunes 12.2 on an iMac (mid 2011) running Mountain Lion. I installed the iTunes update via the Mac App Store today and everything went smoothly - all the Apple Music features are working, everything seems stable. But there's an annoying little glitch: in the "For You", "New", "Radio" and "Connect" tabs I get little grey squares instead of play buttons.

This is what it should look like:

This is what I see EVERYWHERE in Apple Music:

What I've tried so far:

- I removed iTunes and reinstalled it (with the installer downloaded from Apple)

- I reinstalled iTunes with Pacifist (because I remembered that Pacifist supposedly replaces the little files that iTunes scatters all over the HD)

- I deleted the iTunes related caches in Home>Library>Caches

- I repaired permissions

- I restarted the iMac

... but no luck.

I have searched up and down but nobody else seems to have this issue - so I'm kind of at a loss here... I'd very much appreciate any advice on how to proceed.

I know I'm nitpicking since those little squares are not directly affecting my usage of the application, but I'll be honest, they're driving me nuts 😉

PS: maybe I should mention that I upgraded directly from iTunes 11.4 - I wasn't a big fan of iTunes 12 and stayed with 11 until today. I just upgraded because of Apple Music.