Error on Snapshot plugin when trying to backup database


I have a problem concerning Snapshot plugin. I don't know why but until 2 weeks ago, the plugin worked just fine. Today, I tried to make a backup and it gave me the following error :

" Unknown: errno:2 session_write_close(): open(/var/php_sessions/sess_5213e7ee179f64e9ef391e57b255660c, O_RDWR) failed: No such file or directory (2) " followed by the server address. For security reasons, I don't want to write it here.

Can someone help me please?