ADOBE CS6 STILL BUGGIN
I was having the same problem and was finally able to fix it.
I was losing my mind because I had just produced 4 hours of screens and It was generating the same 'could not save file' error.
I refused to give in and began closing windows within adobe and other programs to save ram.
still no luck...
Finally I gave in and clicked the X button to exit the program.
Fireworks prompted me to save the last 2 files I had been working on. With much doubt in my mind I clicked the 'yes' button and magically it saved both files.
So there's your answer.
Simply close the fireworks program and click 'yes' to the Save prompts. It should save them without error.
So much for bug fixes, sometimes you have to H4X 7H47 5H17 40U253LF