[Q] Memory full, but of what? topic

I've had the Galaxy S3 for over an year, and actually I'm on CM11. The phone started to give me the "memory full" alert, but if I go into the settings/memory tab, it shows me that only two thirds of the memory are used. It doesn't let me install apps, and I don't know what to do. Picture related, the memory tab.
Have anyone had the same problem? And if yes, how did you manage to recover from it?

Thanks in advance for your help, and sorry for my bad english, I'm Italian

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