Hello,
I get this when I access to the admin.php :

Warning: session_start() [function.session-start]: open(/mnt/104/sdb/9/e/lordcoy/sessions/sess_62596152a1ca9186aae549798f5a4811, O_RDWR) failed: No such file or directory (2) in /mnt/104/sdb/9/e/lordcoy/Zenphoto/zp-core/admin.php on line 4

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /mnt/104/sdb/9/e/lordcoy/Zenphoto/zp-core/admin.php:4) in /mnt/104/sdb/9/e/lordcoy/Zenphoto/zp-core/admin.php on line 4

Warning: Cannot modify header information - headers already sent by (output started at /mnt/104/sdb/9/e/lordcoy/Zenphoto/zp-core/admin.php:4) in /mnt/104/sdb/9/e/lordcoy/Zenphoto/zp-core/admin-functions.php on line 51

And I can't disconnect from administration can someone help me ?

Try creating the sessions directory on your account... in the /lordcoy/ folder. PHP needs it to store session info.

yeah thanks it works great !

Now I still can't disconnect from administration, any ideas ?

What do you mean by "can't disconnect"?

when I'm in the administration page, I click on disconnect in the upper right corner, and it don't do anything ...

I'm with 1.1.7 version for my theme.

You can close the browser to cancel your session. There have been some improvements in admin login session handling in 1.2. You might want to upgrade.