I’m running the latest version of Trainerday usually on an ipad, sometimes on iphone. Since last week the app starts with a black screen, only showing the setting cog. The only solution is to exit the app and restart it but sometimes have to do this 2-3 times before the workout screen will appear.
I upgraded my iPad to 15.3.1 (not 15.2.1 like you) and did everything I can to try to duplicate this. Also a developer on the team also tried. Maybe you have a huge number of workouts or some crazy workout with a huge number of steps or something. I saw someone create a workout with 4,000 steps in it. You could try removing all your lists from the app, or you plan from the app. Either that or try deleting the app and re-installing.
My ipad auto-updated over the weekend, so maybe this will heip, I’ll test it and let you know. I’ve not got any complicated workouts loaded, just finished week3 of a CJ training plan.