Ok so you have the same prob that I have. Doesn't matter that the phone reports that there's still space, The issue seems to be the security framework Aegis hiding the real used space by it's files. Since we can't do anything bout Aegis, we have no choice but to try free up space.

Best place I found for this is the /home/user/.thumbnails/ folder. You can try the following in Terminal to see how much space is used by the folders inside .thumbnails

You can delete most of the folders in .thumbnails, the OS will regenerate back what is needed. But to make it more safe, you should remove your facebook & twitter accounts from Accounts before deleting those folders taking up huge space.

I had 800mb in .thumbnails folder, after cleaning it up it only ate 100mb++