Unknown: open(/var/lib/php5/sess_

Unknown: open(/var/lib/php5/sess_

Seems like you have a very low-level PHP error related to session handling. You need to make sure PHP5 is installed and working correctly before attempting to use Grav.

More info: https://www.google.com/search?q=Unknown%3A+open(%2Fvar%2Flib%2Fphp5%2Fsess_&oq=Unknown%3A+open(%2Fvar%2Flib%2Fphp5%2Fsess_&aqs=chrome..69i57j69i58.380j0j4&sourceid=chrome&es_sm=119&ie=UTF-8

thanks for the response,

make sure PHP5 work correctly
yes PHP5 work like a charm

strange thing, grav create 3 session
1 - when I visit a site (domain.com)
2 - when I go to admin (domain.com/admin)
3 - after login
the sessions files 1 and 2 are owned by the right user (my user) and the right group: (psacln) so the server can access, rw
and the session 3 is owned by www-data with group www-data, which doesn't exist in my config so the server can't read nor write

note: psacln is a group for the server (this is a standard plesk config)

thanks

After double check, the user and group www-data was already created (but never used by no one of my sites, before grav, don't no why?)
from there the solution was simple
First I cleared all the sessions and the cache, then
1 - I added my user to the group www-data
2 - chmod g+w to the entire cache folder
and that's all!
the site work great :-)

Glad you got it sorted! Would of been a real bear for me to work out what was going on there :)