Unable to install Concrete5 Online.net

I am trying to install concrete5 v5.6.2.1 under online.net and i have encountred the following error message during installation process :

Installer Concrete5
{"error":false} Warning: Unknown: open(/flex/domain/communeetmoi.fr/site/www/files/tmp/sess_moitdtvs372p8dbko66v5kk2l1, O_RDWR) failed: No such file or directory (2) in Unknown on line 0 Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/flex/domain/communeetmoi.fr/site/www/files/tmp) in Unknown on line 0

Can you help me ?

View Replies:
juliandale replied on at Permalink Reply
First of all, ensure that there is a folder called files within the www directory and make that folder writable using folder permissions. You may need your hosting company to assist you with this.
santeroc replied on at Permalink Reply
Thank you for your interrest.

I've spent my day working this problem and I have been able to start my website (http://www.communeetmoi.fr). Now I have another problem.

Sometimes my page appear with :
Warning: session_start(): open(/flex/domain/communeetmoi.fr/site/www/files/tmp/sess_hbdd7ma4i3j0aj71mnh5kikck7, O_RDWR) failed: Input/output error (5) in /flex/domain/communeetmoi.fr/site/www/concrete/startup/session.php on line 36

Warning: session_start(): open(/flex/domain/communeetmoi.fr/site/www/files/tmp/sess_n86kvbsd905di2bl9f2gn78e73, O_RDWR) failed: Input/output error (5) in /flex/domain/communeetmoi.fr/site/www/concrete/startup/session.php on line 36

Do you have any idea ?