I too have suddenly had this problem. It has never occurred before and actually took effect while playing with nothing really changing. I believe the issue was winning a tournament. I had very recently started a new campaign, found an early tournament while on a caravan escort mission, was able to save prior but upon winning the tournament and equipping the won item (a cav lance) it suddenly gave me the "could not save data" while also saying "game saved". Sure enough, the game did not save and would not let me create any further save games. Save on exit did not work either.
I reloaded my previous campaign saves from the game before, each opening fine and also allowing me to update those save files. I tried another new campaign and instantly met the error message of "could not create save data". I verified all my files, I deleted all of the potentially damaged/corrupt saves of the new campaigns and started again, which did allow me to create a save game. I once again found a caravan escort mission and on route, another tournament. I was able to save prior, but upon winning and equipping my new sword, I was met with the "Cannot create save game data" message while also saying "Game saved."
I do not have OneDrive open. I do not have an anti virus running, prior to this suddenly happening today, everything has been fine. I am not running any mods, it does not seem like there were any updates imbetween, this has literally just happened out of the blue. WTF?
EDIT: I continued the game following the caravan despite not being able to save, turns out the caravan got to the first town and then never left... I waited about 2 days. Then I saw a nearby lord, decided to see if he had any of the Netres's Folly quest, when I talked to him, it thought he was my brother and said we were splitting up to find the children? Then bam, got an error message and the game closed.