If you receive this error it can often be because the security policy has changed on the server or you have tried to connect to Exchange with more mobile devices than allowed by your admin. To resolve either of these you'll need to go to your Exchange settings (Through Outlook Web Access or Office365) to Options > General > Mobile Devices. Find the entry which equates to your mobile and delete it. Under Device it will show up as "WP8Tasks" for versions prior to 2.0 and "TasksForWindows" for 2.0 onwards. Delete any other redundant entries here then go back to the tasks app and perform a Sync. It should succeed this time and create a new entry in the devices list.