I saw in the /tmp dir many files created by www-data user, and Drupal can't delete it.

I think that sessions doesn't work too because the same reason.

Any suggestion?

Session files are normally controlled by the web server not applications. The /tmp/sess_* files should be owned by the user that the web server runs under.

Applications use the web server to manage the sessions they create and change.

If you are having problems in Drupal, looking at the error logs will give you a better idea of what is happening.

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Kevin a.k.a. Dweeber