Fatal errors, 2.16.4+15 update errors + XCACHE issue

Since I updated yesterday to 2.16.4 ( and a minute ago to 2.16.5)

I add fatal errors doing the classic update (through the web page).
And this was the very first time it occured.
I add to use shh to run the php command as indicated.
It ended (for both upgrades) xith
Piwik has been successfully updated!

PHP Fatal error:  Call to undefined function Piwik\posix_getgrgid() in /var/www/vhosts/XXXXXYYY.fr/httpdocs/piwik/core/Filechecks.php on line 268

Howver Piwik seams to be ok but NOW:
I have into the piwik stats home page and each pages of piwik an XCACHE error

The following error just broke Piwik (v2.16.5):

XCache: Cannot init


Unknown line 0

just like here: https://forum.matomo.org/t/fatal-error-xcache-cannot-init/12509

My PHP is 5.4.45
The Xcache is coming with the virtual server (OVH) and has always been there.

Thank you

Can you update your PHP version? You should be using 5.6.x. As that post says, XCache is not compatible with current versions of PHP and Piwik.