Not sure why I get these errors every now and then

I have an authentication app using IonAuth. This is the first time I've used IonAuth and every now and then I get this error...

A PHP Error was encountered

Severity: Warning
Message: unlink(/tmp/ci_sessionbd911eef382db5c4a4831662dfe3dd58490b44c3): Operation not permitted
Filename: drivers/Session_files_driver.php
Line Number: 354
Backtrace:
File: /home2/xx/public_html/xx/index.php
Line: 292
Function: require_once


it only happens maybe 1 out of 20 logins, all other times it works fine. Does anyone know why this might be happening? Thanks