Pro Sites continued PHP Warning

I just opened my php_errorlog and it is full of this Warning over and over again.

[01-Dec-2015 00:55:05 UTC] PHP Warning:  session_start(): open(/tmp/sess_fd5f8rvj6sl2u6794s2tsnc293, O_RDWR) failed: No such file or directory (2) in /wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Helper/Session.php on line 29
[01-Dec-2015 00:55:06 UTC] PHP Warning: Unknown: open(/tmp/sess_fd5f8rvj6sl2u6794s2tsnc293, O_RDWR) failed: No such file or directory (2) in Unknown on line 0
[01-Dec-2015 00:55:06 UTC] PHP Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in Unknown on line 0

This is happening over and over again – in the hundreds per day. Please inform. What can I do to fix this Warning?