Enough disk space yet I get "Out of space/memory error"

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http://androidforums.com – After upgrading my Samsung S2 to Jellybean, I have, in my notifications, a "Storage space" warning that some system functions may not work properly. Also, when I try to install any app, I get the error: "Out of space. Free up some space and try again", which gives me a link to Manage Applications. There, I'm shown the memory usage, which is 1.8GB used / 204 MB free. So it seems like the phone is low on *memory* (although I don't know why that is, since in the previous version of Android, ICS, I never had this problem), but the strangest thing is that in terms of *disk spa (General)