Latest Wordpress version with multisite setup in wordpress - not Civi. Upgraded to civicrm 4.6.10 - also existed with civi 4.5.8 - no changes.
I can go to an existing page via the wordpress side (example.com/donate) and the civi - content shows fine. However, if I click contribute on that page (or use the live link from civi back end) I get a 500 server error. No obvious messages with debugging turned on (Are there log files I should review?)
Happens with stripe and paypal and even the manual option of sending by check!
It does not seem to like example.com/donate/?page=CiviCRM&q=civicrm/contribute/transact
Turned on PHP error reporting on my host and was delivered an error message about my google calendar plugin.
Fatal error: Class 'WP_Requirements' not found in /root/www/wp-content/plugins/google-calendar-events/google-calendar-events.php on line 46
Deactivation of the Plugin allows the system to process normally.
Same issue: posted yesterday to developer - no response yet: https://wordpress.org/support/topic/plugin-not-compatible-with-civicrm-profiles