Error on event registration Thank-you page

Using WP 4.24 & CiviCRM 4.6.8

The registration is completed correctly, but the event thank you page gives:

Sorry but we are not able to provide this at the moment.
Could not find valid value for id
Return to home page.

URL of thank you page: http://domain.org/?page=CiviCRM&q=civicrm%2Fevent%2Fregister&_qf_ThankYou_display=true&qfKey=<snip>

From the error log:

Sep 03 12:58:11 [info] $Fatal Error Details = Array
(
[message] => Could not find valid value for id
[code] =>
)

Sep 03 12:58:11 [info] $backTrace = #0 /var/www/domain.org/html/wp-content/plugins/civicrm/civicrm/CRM/Core/Error.php(364): CRM_Core_Error::backtrace("backTrace", TRUE)
#1 /var/www/domain.org/html/wp-content/plugins/civicrm/civicrm/CRM/Utils/Request.php(95): CRM_Core_Error::fatal("Could not find valid value for id")
#2 /var/www/domain.org/html/wp-content/plugins/civicrm/civicrm/CRM/Event/StateMachine/Registration.php(52): CRM_Utils_Request::retrieve("id", "Positive", Object(CRM_Event_Controller_Registration), TRUE)
#3 /var/www/domain.org/html/wp-content/plugins/civicrm/civicrm/CRM/Event/Controller/Registration.php(47): CRM_Event_StateMachine_Registration->_construct(Object(CRM_Event_Controller_Registration), TRUE)
#4 /var/www/domain.org/html/wp-content/plugins/civicrm/civicrm/CRM/Core/Invoke.php(307): CRM_Event_Controller_Registration->_construct("Event Registration", TRUE, "null", NULL, "false")
#5 /var/www/domain.org/html/wp-content/plugins/civicrm/civicrm/CRM/Core/Invoke.php(86): CRM_Core_Invoke::runItem((Array:15))
#6 /var/www/domain.org/html/wp-content/plugins/civicrm/civicrm/CRM/Core/Invoke.php(54): CRM_Core_Invoke::_invoke((Array:3))
#7 /var/www/domain.org/html/wp-content/plugins/civicrm/civicrm.php(1189): CRM_Core_Invoke::invoke((Array:3))
#8 /var/www/domain.org/html/wp-content/plugins/civicrm/includes/civicrm.basepage.php(134): CiviCRM_For_WordPress->invoke()
#9 [internal function](): CiviCRM_For_WordPress_Basepage->basepage_handler(Object(WP))
#10 /var/www/domain.org/html/wp-includes/plugin.php(571): call_user_func_array((Array:2), (Array:1))
#11 /var/www/domain.org/html/wp-includes/class-wp.php(622): do_action_ref_array("wp", (Array:1))
#12 /var/www/domain.org/html/wp-includes/functions.php(877): WP->main("")
#13 /var/www/domain.org/html/wp-blog-header.php(14): wp()
#14 /var/www/domain.org/html/index.php(17): require("/var/www/domain.org/html/wp-blog-header.php")
#15

The issue is also reported by others at:

http://civicrm.stackexchange.com/questions/4793/civievent-error-when-registering-could-not-find-valid-value-for-id
http://civicrm.stackexchange.com/questions/4033/even-registration-emails-not-going-through-registration-produces-error
http://civicrm.stackexchange.com/questions/4983/why-is-the-event-registration-thank-you-page-generating-an-error