If you get an "attention required" displayed in ActiveSync/Windows Mobile Device Center window, a problem occurred while synchronizing. Generally you will see an alert raising from the taskbar which precisely explains the reason of the problem and give you some indications on how to solve it. After that, you must always disconnect and reconnect your mobile device.
If there is no particular explanation regarding the origin of the problem displayed in the alert raising from the taskbar, please send a mail to support [at] birdiesync.com with the log archive built right after the problem occurred, without restarting Thunderbird/Sunbird and without disconnecting your mobile device. You can build the log archive with "Tools/BirdieSync/Archive logs...".