Tasks are saved in filesystem AND registry
Tasks are stored in 3 locations: 1 file system location and 2 registry locations.
File system:
C:\Windows\System32\Tasks
Registry:
HKLM\Software\Microsoft\Windows NT\CurrentVersion\Schedule\Taskcache\Tasks
HKLM\Software\Microsoft\Windows NT\CurrentVersion\Schedule\Taskcache\Tree
So, you need to delete a corrupted task in these 3 locations.