Same

were you able to fix the issue ?
I am getting the same issue for an old diplomacy save which isnt working now

Ok so I'm on mac, and I got this to work. So basically the steps are as follows:
1. Download the current version from dropbox
2. In the steamapps/common/mount and blade/Modules folder create a copy of your Native Modules folder and rename it to something different from just Diplomacy, like "Diplomacy_Temp"
3. Copy and paste the content from the folder you downloaded from Dropbox into the Diplomacy_Temp folder, replacing all files as needed.
4. Start up the module selector, choose your new module (The name will match whatever you name your module folder, in this case "Diplomacy_Temp")
5. Start up the game once, this will create the save folder.
6. Navigate to your savegame folder location for Mac it's in Library/Application Support/MBWarband/Savegames there should now be a new folder that matches the name you named your module (In this case "Diplomacy_Temp") copy and paste the native saves into that folder
7. Run the game it should work now

I did that step-by-step, and it gave me the same error as the OP and I got.