WSUS3.0 SP2 on w2k8r2 stopped syncing, a firewall issue or else?

After March 13th updates to servers I noticed WSUS 3.0 SP2 (on w2k8R2) is no longer able to sync for updates. It actually shows the last successfull attempt on March 13th and then no more (all failed), not really sure what did it.

here is the error in from WSUS console:

TypeInitializationException: The type initializer for 'Microsoft.UpdateServices.ServerSyncWebServices.ServerSync.ServerSyncProxy' threw an exception. ---> System.Net.WebException: The request failed with HTTP status 400: Bad Request.
at Microsoft.UpdateServices.Internal.ClassFactory.CreateInstance(Type type, Object[] args)
   at Microsoft.UpdateServices.ServerSync.ServerSyncLib.GetWebServiceProxyInternal(UpdateServerConfiguration serverConfig, WebServiceCommunicationHelper webServiceHelper, Boolean useCompressionProxy)
   at Microsoft.UpdateServices.ServerSync.ServerSyncLib.GetWebServiceCompressionProxy(UpdateServerConfiguration serverConfig, WebServiceCommunicationHelper webServiceHelper)
   at Microsoft.UpdateServices.ServerSync.CatalogSyncAgentCore.RetrieveSubscriptionData()
   at Microsoft.UpdateServices.ServerSync.CatalogSyncAgentCore.ExecuteSyncProtocol(Boolean allowRedirect)

in firewall both WSUS related rules are allowed, all clients can connect for updates (over SSL) without trouble, same goes for remote admin consoles (working in plain and SSL mode both).

It sounds like it sends a bad HTTP request (error 400), but why? what am I missing here?

I tried installing KB2734608, but it didn't help in resolving the sync fail problem. it still persists.

just to rule out Firewall issues, I disabled it and it still fails, so re-enabled it back on since it is unrelated.

any ideas?

EDIT:

last server updates installed on March 13th were:

KB2791765

KB2806748

KB2809289

KB2807986

KB890830