This happens when you try to acces an external class on a frame that doesn't have the import statement for the class. ie:
The solution is to put the import statement (also) on frame 5.
Today is turning out to be a real Friday the 13th! Everything that could go wrong, goes wrong... 
Swift3D can't export object with bitmap textures for vector output, while I desperately needed that. Other than that am I stuck on a weird problem with Flash and not showing some of my MovieClips, not showing the entire SWF in the browser at times AND Flash stopping the playhead at the wrong framelabel...
This day better be over soon! Aaaaaaarrrgghh!!! :p
You should use the decimal character references for these characters to use them in your XML. So instead of writing รก, you should write '& # 2 2 5 ;' (remove the spaces). This way Flash will display the characters correctly.
A list of the char references can be found at: http://www.bbsinc.com/iso8859.html
EDIT: Another way of solving this is setting the XML encoding to UTF-8.