Hi,
Just had the same problem. All worked fine until we implemented Firewall rules via policies last week. I found this link "http://scug.dk/blogs/configurationmanager/archive/2009/09/22/hmmm-the-firewall-once-again.aspx". I only changed this FW rule on the DP server, not the primary site server and everything started working again.
Very strange because I can not find any documentation (yet) from MS which states that we need this rule.
Hope this helps.
Rgds, Ton