Popover plugin gives 'headers already sent' warning, makes it unable to log into the wp-ad

Hi,

I've Installed the popover plugin from your site. It works and does what i want it to do.

My problem is as follows:
If I log out of my wp-admin and later on try to log onto the wp-admin again, wp wont let me login. I have to remove the plugin from the plugins folder, log in and than add the plugin back to the plugins folder again.

I have deactivated all other plugins to see if there were any conflict. The problem persisted and there are no conflicts.

These are the errors I get when I try to log in:

Strict Standards: Static function Popover_Rules_Rule::add() should not be abstract in /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/media/plugins/popover/popoverincludes/includes/functions.php on line 453

Warning: Cannot modify header information - headers already sent by (output started at /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/media/plugins/popover/popoverincludes/includes/functions.php:453) in /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/wp-login.php on line 415

Warning: Cannot modify header information - headers already sent by (output started at /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/media/plugins/popover/popoverincludes/includes/functions.php:453) in /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/wp-login.php on line 427

Warning: Cannot modify header information - headers already sent by (output started at /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/media/plugins/popover/popoverincludes/includes/functions.php:453) in /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/wp-includes/pluggable.php on line 678

Warning: Cannot modify header information - headers already sent by (output started at /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/media/plugins/popover/popoverincludes/includes/functions.php:453) in /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/wp-includes/pluggable.php on line 679

Warning: Cannot modify header information - headers already sent by (output started at /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/media/plugins/popover/popoverincludes/includes/functions.php:453) in /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/wp-includes/pluggable.php on line 680

Warning: Cannot modify header information - headers already sent by (output started at /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/media/plugins/popover/popoverincludes/includes/functions.php:453) in /Users/simonscholten/Documents/bdm/projects/MeidenVanHollandNL/Project/wp-includes/pluggable.php on line 896

How can I fix this problem?

Thank you for your time.