I hate Google in general for their bad business practices but Google Search is truly amazing. I did a search
for "Open_data_file_savely" (I liked the sound of that for some reason 
) and I found a thread from six and a half years ago started by somebody who apparently had the same problem:
http://www.ootpdevelopments.com/boar...ng-glitch.html
Unfortunately, there is no indication that the problem was ever resolved, even though Andreas Raht himself responded. Your problem refers to a file in the saved games folder, as shown below. Something is preventing the game from reading that file (or maybe it was deleted?).
Your best bet is to post this problem in Tech Support or, if you're feeling lucky, send a PM to Andreas referring to this thread and that old one and ask him if he ever found a solution. Good luck.