The iTunes library file cannot be saved.

I received a message "The iTunes library file cannot be saved. There is not enough memory available."

I received it several time, once it also added:
(error -50)

What does that mean ? Not my Music Folder in iTunes, is it ?
I know it can handle 32.000 songs.

I have, it says at the bottom:
3716 songs, 240 days, 24.64 GB
------
I also received later a message telling me to re-install iTunes.
How do I do that ?

Also since I have version 8.2.1, instead of re-installing this, can't I just upgrade to the latest version ?
In other words is iTunes the "basic" application and all the upgrades are just adds-on (but the base remains the same) or is all upgrade a complete application in itself and thus upgrading would be the same as re-install (= I'm installing a brand new version) ?