Although leaving TASK unchecked will temporary fix the problem, but that means you cannot sync TASK.
If you want to fix it properly, please follow these steps:
1: unplug usb cable if you are still connecting it with your WINDOWS MOBILE
2: Open MS Outlook (I am using 2007 as an example), go to File- > New -> Outlook Data File , and create a new PST file (what ever name/ location)
3: In outlook, Tools ->Account settings, then click Data Files tab, click the new PST file that you had just created, and set as default , then restart outlook
4: delete activesync partnership in both Windows Mobile and in your Desktop
5: Plug the WM to the USB port, and it should sync properly again.
6: restore the backup by using MS Outlook’s Import and export function

http://forums.microsoft.com/windowsmobile/showpost.aspx?postid=2868305&isthread=false&siteid=65&authhash=e3d5a913a973750eacff18ceca23dd05475f13c5&ticks=633389876668929937&sb=0&d=1&at=7&ft=11&tf=0&pageid=1