Hi, 
  
I came across this issue last week. Have you configured a FSP? If not, then do that. Also, uncheck "Clients check certificaterevocation list (CRL) for site systems". You can find this setting in: 
  
Administration--->Hierarchy Configuration--->Sites 
  
Just highlight "Sites" and click "Properties" in the ribbon uptop. 
  
Let me know if this helped you.