Getting blank page while trying to login to admin panel

Hi there,

I am currently facing a problem with my wordpress blog https://www.seo-coaching.net. When trying to log in a blank page appears in the browser (Firefox and IE). It is probably a problem with PHP and when I empty my cache, the login page reappears.

This problem first occurred after I had installed my new theme “novocane” and the plugin SEOpressor. Maybe there is a link to that.

I already tried to adjust the wp-config.php file and added following lines:

define('WP_MEMORY_LIMIT', '128M');
define('WP_CACHE', false);

However, this didn’t solve the issue. After that I turned on error reporting for PHP with:

ini_set('display_errors',1);
error_reporting(E_ALL|E_STRICT);

Follwing output was generated:

Strict Standards: Declaration of Walker_Category_Checklist::start_lvl() should be compatible with that of Walker::start_lvl() in /is/htdocs/wp1048090_8JCEI180MP/seo-coaching/wp-admin/includes/template.php on line 215

Strict Standards: Declaration of Walker_Category_Checklist::end_lvl() should be compatible with that of Walker::end_lvl() in /is/htdocs/wp1048090_8JCEI180MP/seo-coaching/wp-admin/includes/template.php on line 215

Strict Standards: Declaration of Walker_Category_Checklist::start_el() should be compatible with that of Walker::start_el() in /is/htdocs/wp1048090_8JCEI180MP/seo-coaching/wp-admin/includes/template.php on line 215

Strict Standards: Declaration of Walker_Category_Checklist::end_el() should be compatible with that of Walker::end_el() in /is/htdocs/wp1048090_8JCEI180MP/seo-coaching/wp-admin/includes/template.php on line 215

Warning: Cannot modify header information - headers already sent by (output started at /is/htdocs/wp1048090_8JCEI180MP/seo-coaching/wp-content/plugins/seo-pressor/seo-pressor.php:45) in /is/htdocs/wp1048090_8JCEI180MP/seo-coaching/wp-includes/pluggable.php on line 890

Can anybody help?

Thank you and regards.