Error message 'The selected task "{0}" no longer exists. To see the current tasks, click Refresh’

A lot of customers complained the following:
When they open up the Task Scheduler, they get a message that says:
The selected task "{0}" no longer exists. To see the current tasks, click Refresh.

 The mechamism task scheduler uses: it has a copy of tasks in registry and a copy of them in task files on disk. If somehow the tasked stored in registry and on disk are not in sync, users will encounter such issues as reported.

Tasks are stored in C:WindowsSystem32Tasks

In Registry, related branches are located in:
HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionScheduleTaskCacheTasks

Tasks tree is stored in:
HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionScheduleTaskCacheTreeMicrosoft

So the general troubleshooting steps are starting from checking these locations to see if anything is broken or inconsistent.

Reference:

 http://social.technet.microsoft.com/Forums/en/w7itproinstall/thread/69afe198-3f97-406a-8d91-6a17504fb5b5

http://social.technet.microsoft.com/Forums/en-IE/w7itproinstall/thread/273368d4-ed12-4107-9d52-34759f2b53aa

http://social.technet.microsoft.com/Forums/en-US/w7itprogeneral/thread/1f677dd3-bdb7-4650-9164-d8e2c66b7708

http://social.technet.microsoft.com/Forums/en/w7itprogeneral/thread/d584f7e4-4e4c-45e8-8bce-39414914d674

http://social.technet.microsoft.com/Forums/en/w7itprogeneral/thread/a9e485d3-06ba-4038-a9b8-1b58e73ea4dd

http://social.technet.microsoft.com/Forums/en-US/w7itprogeneral/thread/360fde7c-ec4e-41f2-ae74-8949218e7a06

http://social.technet.microsoft.com/Forums/en/w7itprosecurity/thread/8e779ed1-7200-482e-a3a4-1e0ba8bf17cc

http://social.technet.microsoft.com/Forums/en/w7itproperf/thread/94bb210a-b291-41ad-a745-d4a9d8cc77dc

http://social.technet.microsoft.com/Forums/en/w7itprogeneral/thread/344f6815-0878-43ab-8997-33790b889541

http://social.technet.microsoft.com/Forums/pl-PL/w7itproperf/thread/43712af2-5731-4177-9d45-31d9b5b3240d