I have the same problem when trying to download to my Garmin Oregon 450 from my iMac. The problem just started and i contacted GARMIN who said this has just come up and is a GEOCACHING.COM application problem.
It seems the driver for USB connectivity may be the problem. I re downloaded and installed Communicator 4.1.0.0 and the problem still persists.
I tried using SAFARI and FIREFOX but have the same problem.
Can someone from Groundspeak/Geocaching.com help us out as the Geocashing is now unavailable to me until this is fixed.