This is a bug that got so bad with CS6 that I had to kick out Fireworks out of my workflow. Adobe ignores this problem completely. It is persistent since 5 years. They do not care about it, they never will. You are lucky that you only get the Internal Error. For me the file simple closes, Fireworks crashes without any error message and sometimes the file is completely destroyed and cannot be reopened. Fireworks is dead because of this.
I resolved this issue by renaming two files at: C:\Documents and Settings\your.name\Application Data\Adobe\Fireworks CS6\English:
Fireworks CS6 Panel Prefs.xml
Fireworks CS6 Preferences.txt
Changing them to:
Fireworks CS6 Panel PrefsOLD.xml
Fireworks CS6 PreferencesOLD.txt
After renaming these files, Fireworks operates normally again.
I am running Windows XP Professional, so the file path may be slightly different if you are on a different version of Windows, but the files should be somewhere near the same path.
Hope this helps!