Hallo,
since I have the patch for CalDAV installed there is a very annoying prompt for username and password of the calendar on TB startup.
Username and password are stored by Thunderbird so I have to click just OK, but there's no such thing as never promt again or so..
Before this patch I was never asked for password, just at creation/adding of the calendar..
caldav patch: prompt for user and password
I think that upgrading to Lightning 0.7, upgrading BirdieSync and reinstalling the new patch should solve your issue.
Birdy