Several reports in the Internet with such error:

[22-Mar-2019 21:35:20 Africa/Tunis] PHP Warning:  Unknown: Failed to write session data (memcached). Please verify that the current setting of session.save_path is correct (PERSISTENT=2 10.10.10.10:11211) in Unknown on line 0
[22-Mar-2019 21:35:21 Africa/Lagos] PHP Warning:  Unknown: Failed to write session data (memcached). Please verify that the current setting of session.save_path is correct (PERSISTENT=2 10.10.10.10:11211) in Unknown on line 0
[22-Mar-2019 13:35:21 America/Los_Angeles] PHP Warning:  Unknown: Failed to write session data (memcached). Please verify that the current setting of session.save_path is correct (PERSISTENT=2 10.10.10.10:11211) in Unknown on line 0

But no real solutions worked for us. Here is our setup:

Several web servers servers (using php-fpm – fastcgi) connect to a single memcached server to share sessions.

The PHP configuration (php.ini) is as simple as that:

....
session.save_handler = memcached
session.save_path = "PERSISTENT=2 10.10.10.10:11211"
....

This is how you can enable PHP sessions to be stored in the memcached server with IP 10.10.10.10 and tell the driver to use persistent connections. Note we use the binary PHP module memcached, not memcache, which is the old driver.

The persistence is handled by the PHP module, not the library, which it wraps and it is per process not per all PHP fpm (fastcgi) processes.
Apparently, sometimes this functionality just get corrupted and many of those errors begin to appear in the logs – most of the time a reload of the php-fpm after a long period of time. Even a restart did not help, so the real solution, which always works for us is

to change the ID of the “PERSISTENT=”

so just change to:

session.save_path = "PERSISTENT=234 10.10.10.10:11211"

No matter of the ID just some new one, not used previous time! And everything continues smooth!