The team I'm managing has become part-time, meaning there are now significant gaps of "Unavailable" sessions. Still, when I try and change the calendar training sessions I do have, it then makes those days unavailable, meaning my players aren't training for weeks!
I've played Football Manager for years and have never had a club turn part-time, so not an easy one to reproduce but try editing training sessions for part-time clubs, I'm not sure if it's happening to all of them.
Only a little one, I don't know if it's just happening on my save (Farsley) but it's saying that my favoured personel have had far more appearances than they have!