Earlier this year, the Google folks announced a change to how applications like FeedDemon should authenticate with Reader. Later this week, the old authentication system will be dropped.
FeedDemon has used the new authentication method ever since version 184.108.40.206, which was released several months ago. If you’re using an older version, you’ll want to update to the latest version right away – otherwise, synchronization may no longer work for you.
PS: Sorry for the prolonged silence here – my family is in the middle of a move which turned out to be far more complicated than we planned!