The most common causes of this error message are

1. Empty frames in the main timeline and/or library symbols’ timelines.
2. Invalid characters in TLF textfields.  The new file format handles  most invalid XML characters, but some characters (such as ETX  characters) are not parsed correctly.
3. The FLA contains corrupt assets.

Fix empty frames tags in main timeline:

1. Make a back up of your FLA in another location.
2. Change the extension of the FLA to .zip so you can unzip the FLA into its folder structure (XFL format).           
   Note: On a Mac, using the default unzip  utility sometimes only unzips the file into a .cpcg file. To workaround  this, use a utility like StuffitExpander or 7zip on the Mac.
3. Open DOMDocument.xml and search for an empty <frames/> tag.           
   a. Replace all occurrences of <frames/> with:           
   <frames>           
   <DOMFrame index="0" keyMode="9728">           
   <elements/>           
   </DOMFrame>           
   </frames>           
   b. Save DOMDocument.xml           
   c. Open the .xfl file inside the unzipped folder from step 2.           
   d. If Flash opens the file fine, you are done.  You should verify the  symbols and layers that you replaced the empty <frames/> tags in  still look correct.  You should re-save the file as a new FLA file in a  different location.           
   
   Fix empty frames tags in library symbols' timelines:           
   If there are no empty <frames/> tags in the DOMDocument.xml file  or the .xfl file still doesn't open in Flash, there is likely some  asset(s) causing the problem. You must narrow down the bad asset(s) with  the following steps:
4. Repeat steps 3 with XML files within the LIBRARY folder.  Use a text  editor or any other tool that allows you to search for strings in  multiple files/directories.  Notepad++ is a free text editor that has  this feature.  Search for <frames/> within the entire folder.  On  Mac, you can also use the ‘grep’ command in a terminal window to find  the files that contain the bad <frames/> tags.           
   
   Fix TLF Text:           
   If there are no empty <frames/> tags in XML file(s) or the .xfl  file still doesn't open in Flash, check if any TLF objects have invalid  characters.
5. To find all TLF objects in your FLA, search for  “<markup><tlfTextObject” in the DOMDocument.xml file and all  XML files within the LIBRARY folder and sub-folders.
6. Check if any of these TLF objects have invalid characters like the  ETX character.  Notepad++ will show such characters.  Remove these  characters and check if your file opens.

Unfortunately, there is no easy way to identify corrupt assets  in a  FLA file.