Known "issue" without any solution.
Fireworks is incredibly buggy, and has memory issues which lead to frequent freezes, crashes, and errors. There is really no fix to this, and it almost makes the product unusable (a shame since it's the only Adobe product created for screen design).
I suggest editing your fireworks preference file and enabling auto-save. Just do a google search and you should find some instructions.
Also, if you notice Fireworks acting weird in ANY way, i.e slowing down or glitching, save your work immediately and restart fireworks.
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