usual suspects/fixes for me are (usually in this order):
start > settings > connections > usb to pc > uncheck enable advanced network functionality.
soft resets/computer restarts
delete partnership in wmdc
uninstall/reinstall wmdc
hard resets (this is usually my last resort)
it has become so annoying for me that i usually dont sync that often (sadly i dont get new phone numbers often enough for that to be a problem, haha, but updating the calender more often would be nice)
mail2web is also always an option
also, google searching on error codes is usually helpful:
http://www.google.com/search?q=0x860...ient=firefox-a
from briefly reading through those results, it looks like you need to delete the partnership in wmdc