Quite suddenly the other day, my BlackJack with Windows Mobile 6 stopped syncing with my Exchange mailbox. The error read "Attention Required" and although I gave it a lot of attention, I was unable to get things working by re-entering the server info and my password.
Finally, I went so far as to de-select all of the options to sync - Contacts, Calendar, E-mail, and Tasks. Clearing these checkboxes will delete the data on your device when you apply the change. I then went back and re-enables all of the checkboxes and ActiveSync started pulling down data and syncing properly again.