This is a bit late, but if you are still having issues or if anyone else has this problem and comes across this thread, here are some things you can try;
1) Saving in game on a different slot. Most likely will not work.
2) Go to C:\Users\USERNAME\Documents\Mount&Blade Warband Save Games and there should be a folder for the Module you are playing (for example, if your issue is in Vikings Conquest, that's the folder) right click that folder and change its name to something like Vikings Conquest_bak or Vikings Conquest v2. Anything. Next, create a new folder called Vikings Conquest. That's right, it should be named as if it were the original one but should have nothing in it. Now, load up your game (assuming you can't load up an old save but you just can save a new save) and try saving. If it is successful, delete v2 folder and you are done. If it works, drag any old saves one at a time into this new folder and test. If one of them triggers unable to save errors, that save is corrupted.
3) Repair missing folders. This has been known to solve the issue for some who have had this issue in Warband. [url=https://scottiestech.info/2014/11/11/repair-missing-user-folders-in-windows-7]https://scottiestech.info/2014/11/11/repair-missing-user-folders-in-windows-7
4) Be sure your anti-virus is NOT blocking the game (this means adding Mount & Blade as an exception to whatever anti-virus program you are using). This is absolutely known to cause saving issues.
5) Be sure you are running the game as admin and windows 7 compatibility and have FULL ACCESS to the Documents location. You can check if you have full access to Documents and your save files (read only etc) by right clicking the save folder and going to properties and seeing if it says "read only".
If you are still encountering issues after the troubleshooting listed here, please post and I can try to help you further. It could be that you lost access rights to the entire Documents area or something like that, or you just need to verify the game as something is broken or your save file just got corrupted (if you aren't starting new).