I believe the correct permissions are 0770
And the owner should be the web server, in my case: nginx
chown -R nginx:nginx /var/lib/php/session
Since my original posting I have determined the actual problem is the php upgrades.
When php is upgraded, the owner is changed to 'apache' which is php's default setting.
This is a known issue and causes problems to any one whose web server is not 'apache'. Until such time as php recognizes the there is life beyond apache and leaves the 'owner' as is if it exists, this is going to cause problems.
- If you don't have an 'apache' user/group then add it: useradd -s /sbin/nologin apache
- Make 'apache' the owner: chown -R apache:apache /var/lib/php/session
- Add your web user to the apache group: gpasswd -a psacln apache
- For others: gpasswd -a [web-user] apache eg. gpasswd -a nginx apache
The next time php is upgraded you won't find your site inaccessible.
I suspect this will be addressed by php at some point but it hasn't been as of this week.
I don't understand why MagenX didn't provide an explanation and solution rather than a somewhat meaningless comment.