I get this current error after deleting maintenance.flag in the root folder. I was getting that error after a process in the Magento Connect Manager was interrupted while the check box for maintenance was selected and attempting to go back to the admin area. That is the reason I deleted the maintenance.flag file in the first place, to try to fix that previous issue.
Something to note now. Every time I refresh the admin page trying to access it, I get a different error log. Most of the time the error logs are the same (listed below), but sometimes there are a few extra lines in the error log for no apparent reason.
What are the steps I should take now to fix this issue?
a:5:{i:0;s:61:"Mage registry key "_singleton/tmcore/observer" already exists";i:1;s:1606:"#0 /home/amelkuvn/public_html/pixeltini.com/app/Mage.php(223): Mage::throwException('Mage registry k...')
#1 /home/amelkuvn/public_html/pixeltini.com/app/Mage.php(477): Mage::register('_singleton/tmco...', false)
#2 /home/amelkuvn/public_html/pixeltini.com/app/code/core/Mage/Core/Model/App.php(1316): Mage::getSingleton('tmcore/observer')
#3 /home/amelkuvn/public_html/pixeltini.com/app/Mage.php(448): Mage_Core_Model_App->dispatchEvent('controller_acti...', Array)
#4 /home/amelkuvn/public_html/pixeltini.com/app/code/core/Mage/Core/Controller/Varien/Action.php(527): Mage::dispatchEvent('controller_acti...', Array)
#5 /home/amelkuvn/public_html/pixeltini.com/app/code/core/Mage/Adminhtml/Controller/Action.php(160): Mage_Core_Controller_Varien_Action->preDispatch()
#6 /home/amelkuvn/public_html/pixeltini.com/app/code/core/Mage/Core/Controller/Varien/Action.php(407): Mage_Adminhtml_Controller_Action->preDispatch()
#7 /home/amelkuvn/public_html/pixeltini.com/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(250): Mage_Core_Controller_Varien_Action->dispatch('login')
#8 /home/amelkuvn/public_html/pixeltini.com/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#9 /home/amelkuvn/public_html/pixeltini.com/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#10 /home/amelkuvn/public_html/pixeltini.com/app/Mage.php(684): Mage_Core_Model_App->run(Array)
#11 /home/amelkuvn/public_html/pixeltini.com/index.php(87): Mage::run('', 'store')
#12 {main}";s:3:"url";s:6:"/admin";s:11:"script_name";s:10:"/index.php";s:4:"skin";s:5:"admin";}