session_write_close(): Failed to write session data using user defined save handler

Hello,

I want to allow concurrent requests from a client for simultaneous AJAX calls. 
In each php function called, I start my code with session_write_close() to unlock the session. But I randomly get this message : 

Code:

session_write_close(): Failed to write session data using user defined save handler.

I'm using redis and everything work fine. Just getting this message sometimes in my logs.

Any idea?
Cheers