The other day I ran across a little problem that was somewhat hard to track down. 

 

Here's the core scenario - web app set to use SSL with various calendar lists sync'd to Outlook.

 

Everything was working as expected.  Then suddenly it all stopped working.  Outlook would report a generic HTTP 0 error.  New attempts to connect the lists to Outlook would also fail.

 

I went through a variety of diagnosis steps and nothing was popping up.  The site was operational, the certificates were valid, nothing had changed from client to server... tried to see if there was a difference between Outlook 2007 and 2010.  It was all the same generic error.

 

Coincidentally, I wanted to set that problem aside and fix another issue.  I happened to open up SPD and noticed that it threw up a dialog about not being able to validate the CRL for the certificate that was associated with the site.

 

Bingo!  A clue.

 

Sure enough, the CRL had expired.

 

On updating and publishing the CRL, everything started working again.

 

Net net: stssync will fail quietly if the CRL from the CA has expired. The user is not notified and requests are terminated.

 

Don't forget to double check the expiration of the CRL!