Mac Mini is no longer playing movies in iTunes

I have a Mac Mini connected to my TV, and use it to play content. For many years this has included movies downloaded from the iTunes store under our Family Sharing accounts. Recently none of the movies will play back, although purchased music will do. After pressing play on the movies, the screen goes a blank black or beige and nothing further happens. I have reauthorized the accounts and I don't think this is the problem.

I'm running OS10.13.6 and iTunes 12.8.1.2 on a 2.3GHz Mac Mini dating from 2011. It has 4GB ram and 100GB still available on its 500GB hard drive.

Any ideas?

Please don't tell me that the Mac Mini is now too old to play iTunes movies....