As Andrey asked, perhaps we should start with the file size - number of tasks and file kB. Do you have a lot of linked images?
Maybe a corruption in the file? It might be worth looking at the .xml in a text editor (or one which highlights xml keywords - I think Notepad++ does that, but it might need an xml plugin) to see if you can spot the pattern of the file breaking - That happened to me once, several years ago, and I was able to see where it went wrong quite clearly when I saved as an MLO xml file and looked at it in a text editor.
Stéphane