I had the same Problem and found a solution that works for me.
First, copy your corrupted FLA into a new folder. Rename your FLA to [Your File].zip. Then Open that ZIP File with WinRAR and press Alt + R to repair the Archive. Extract the content of the fixed new ZIP file into another folder.
Now, open the "DOMDocument.xml" in Safari. In my case, Safari throws a XML Error. I opened that XML with an Editor and found a non UTF-8 Char at the Line where Safari throwed the error. I deleted that Char, saved the XML and opened the XFL File. Everything works fine.
Where did that non UTF-8 Char came from? In my case, i copied same Text from Powerpoint into a component property field. And everybody should know, that copied text from MS Products comes with a lot of garbage, that nobody needs. Flash saves your texts with that garbage in a xml and your FLA is coruppted.
