I have the same problem. i am playing via the app on my apple smartphone. I am managing 2 teams and with one of those teams the players who are training are stuck there. i cant select or deselect them and neither can i select new players for training. The team i have this problem with is Ajax in the "OSM prijzenleague". this is slot 2 of my managerprofile Erk030280.
I've downloaded version 4.4.15 of OSM and problem is solved for me.