WARNING: end of tag DefineFont3 is @55343, should be @58327
Build halted with errors (swfmill)."
To make a long story short, this was caused by a textfield that didn't embed any characters. Thanks to Philippe for providing the answer.
So, If you get this error (the numbers after the @ will probably be different) check your movieclips for textarea's without embedded characters.
And to make this short story a little longer: (for those of you who are still reading ;)
My problem in finding this was that I used 1 font library, and had all my textarea's set to use that as the font...
But I still had 2 buttons, with the same font, but not using the font library.
So the error occured when I changed the font in the library.
Now the 2 buttons didn't share a font that other textarea's used, and had characters embedded for them.
And due to the fact that it wouldn't compile, I couldn't visually verify all textarea's to be the same at runtime.
Click here to see my thread about it on Flashdevelop.org.