I recently ran across this same issue, but it was not due to the fast spanning tree protocols. Our IT department added 802.1x authentication on the wired ports of all our switches, but the time out to fail back to MAC authentication was just longer than the 27 seconds @george1421 referenced in his post. We had a handful of hosts work, but a large majority failed. Our IT department had to shorten the 802.1X authentication timeout in order to make this work for us.