I was playing a game vs Athena a few weeks ago at patch level 1.15 and ran into an issue in which the turn could not complete. It didn't have anything to do with loading or unloading troops from or onto transports. I sent the turn to support@ and got a reply that it would run with the then 1.16b3 patch and that I should try that. So I installed that and could continue the game.
Pocus does state though that there is an incompatibility in the save formats, but does not go into what that incompatibility is or how it could affect moving a running game from one patch level to another, upwards or downwards.
Pocus wrote:Knows though that it is not save compatible.