exception 'Mage_Core_Exception' with message 'Mage registry key "_singleton/admin/session" already exists'

[209DS]

I have the following problem with the Aoe_Scheduler and the heartbeat function.
I use Magento Version 1.9.3.4 and PHP-Version: 5.6.30

Can somebody help me with this?

exception 'Mage_Core_Exception' with message 'Mage registry key "_singleton/admin/session" already exists' in /home/server/public_html/app/Mage.php:595 Stack trace: #0 /home/gibbon/public_html/app/Mage.php(223): Mage::throwException('Mage registry k...') #1 /home/gibbon/public_html/app/Mage.php(477): Mage::register('_singleton/admi...', Object(Mage_Admin_Model_Session)) #2 /home/gibbon/public_html/app/code/community/Aoe/Scheduler/Model/Schedule.php(560): Mage::getSingleton('admin/session') #3 /home/gibbon/public_html/app/code/core/Mage/Core/Model/Abstract.php(316): Aoe_Scheduler_Model_Schedule->_beforeSave() #4 /home/gibbon/public_html/app/code/community/Aoe/Scheduler/Model/ScheduleManager.php(267): Mage_Core_Model_Abstract->save() #5 /home/gibbon/public_html/app/code/community/Aoe/Scheduler/Model/ScheduleManager.php(165): Aoe_Scheduler_Model_ScheduleManager->generateSchedulesForJob(Object(Aoe_Scheduler_Model_Job)) #6 /home/gibbon/public_html/app/code/community/Aoe/Scheduler/Model/Observer.php(43): Aoe_Scheduler_Model_ScheduleManager->generateSchedules() #7 /home/gibbon/public_html/app/code/core/Mage/Core/Model/App.php(1357): Aoe_Scheduler_Model_Observer->dispatch(Object(Varien_Event_Observer)) #8 /home/gibbon/public_html/app/code/core/Mage/Core/Model/App.php(1336): Mage_Core_Model_App->_callObserverMethod(Object(Aoe_Scheduler_Model_Observer), 'dispatch', Object(Varien_Event_Observer)) #9 /home/gibbon/public_html/app/Mage.php(448): Mage_Core_Model_App->dispatchEvent('default', Array) #10 /home/gibbon/public_html/shell/scheduler.php(519): Mage::dispatchEvent('default', Array) #11 /home/gibbon/public_html/shell/scheduler.php(37): Aoe_Scheduler_Shell_Scheduler->cronAction() #12 /home/gibbon/public_html/shell/scheduler.php(545): Aoe_Scheduler_Shell_Scheduler->run() #13 {main}Set-Cookie: adminhtml=cu52j4e3aivu72va47ipe6s2s6; expires=Wed, 15-Nov-2017 21:05:03 GMT; Max-Age=3600; path=/; HttpOnly Expires: Thu, 19 Nov 1981 08:52:00 GMT Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0 Pragma: no-cache Content-type: text/html; charset=UTF-8