Cannot Send Session Cache Limiter after installing PopUp Pro Plugin

Hi there,

It seems that I have hit a little snag while installing Popup Pro from the WPMU Dev Dashboard.

As soon as I activate it this is the error displayed:
Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/xxxxxxx/public_html/dev/wp-content/plugins/popover/inc/external/wpmu-lib/inc/class-thelib.php:145) in /home/xxxxxxx/public_html/dev/wp-content/plugins/popover/inc/external/wpmu-lib/inc/class-thelib.php on line 145

I have tried to switch to a wordpress default theme and I get a page with lots of error information I have a screenshot but not sure how to attach it and it contains sensitive information.

here is a text dump of it:

Warning: session_start(): open(/tmp/sess_67fc9011d1d5e5651a8967c301f5e7a9, O_RDWR) failed: Permission denied (13) in /home/..../public_html/dev/wp-content/plugins/popover/inc/external/wpmu-lib/inc/class-thelib.php on line 145

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/..../public_html/dev/wp-content/plugins/popover/inc/external/wpmu-lib/inc/class-thelib.php:145) in /home/...../public_html/dev/wp-content/plugins/popover/inc/external/wpmu-lib/inc/class-thelib.php on line 145

Warning: Cannot modify header information - headers already sent by (output started at /home/.../public_html/dev/wp-content/plugins/popover/inc/external/wpmu-lib/inc/class-thelib.php:145) in /home/....../public_html/dev/wp-includes/pluggable.php on line 1174

Warning: Unknown: open(/tmp/sess_67fc9011d1d5e5651a8967c301f5e7a9, O_RDWR) failed: Permission denied (13) in Unknown on line 0

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

What should I do?

WPMUDev support is active on my wordpress site if you need to take a look.
Wordpress Version: 4.6.1

Thank you guys.

Cheers,
Colin.