Maybe you are trying to save to a read-only area of the disk? Or maybe your disk is full? Try saving to your desktop...that is usually an area that everybody can write to. If that fails as well, you may be out of disk space.
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I had that happen to me before. I was working on a seemingly uncomplicated problem (realistic walking) when I decided to try and save. It didn't let me save it on that name, so I made a new name up. I still don't like it. I think shutting down your computer, and restarting it should fix any problem. I hope.
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Try selecting a different folder. i had this problem, but i think you have to double click (open) other folders until it is highlighted. make sure it's not read-only. (i.e: Adobe flash) or something. (I accidentally put it in a folder named this)
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