Hello, is there any mods installed to your game? or have you installed any mods before? If not can you please send us a save file where we can reproduce the issue? You can send the save file to me directly via DM or upload it to the http://upload.taleworlds.com:8080/ address. Please paste the link to this thread and your username of the forum in your description so i can identify it.
It is becouse lords\captains on mounts in squad. Try to kill his horse.
Mods are the best way to add all the features you mentioned, then players can pick and choose which they want .. plus these feature mods will be developed quicker.
Just FYI - many of us with this "can't create save file" issue have found that we can save certain versions of the game. Personally I can play up to and including vers.1.41, but not vers.1.42 or vers.1.43 - so you could try to change the version you're using and see if it is possible to save there. It's a bit of a hassle as each variation usually takes 5-10GB of download to change. But if you REALLY want to try the game, that might be an option.