I am getting these errors when PopUp Pro is activated:

I am getting these errors when PopUp Pro is activated:

The plugin generated 357 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

unction.session-start]: open(/home/content/19/11937719/tmp/sess_82lmeepmtdkld6itkfp7noddt1, O_RDWR) failed: No such file or directory (2) in/home/content/19/11937719/html/wp-content/plugins/popover/inc/external/wpmu-lib/inc/class-thelib-base.php on line 68

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/content/19/11937719/html/wp-content/plugins/popover/inc/external/wpmu-lib/inc/class-thelib-base.php:68) in /home/content/19/11937719/html/wp-content/plugins/popover/inc/external/wpmu-lib/inc/class-thelib-base.php on line68

Warning: Cannot modify header information - headers already sent by (output started at /home/content/19/11937719/html/wp-content/plugins/popover/inc/external/wpmu-lib/inc/class-thelib-base.php:68) in /home/content/19/11937719/html/wp-includes/pluggable.php on line 1196

How do I fix this?

Thanks!