Alert_Deny for cookie Decryption Error

Hi,

Currently we enable cookie session1 at fortiweb proxy. Security mode is Encryption . my understanding cookie session exchange in fortiweb as per below.

1.User ____Get_____> Fortiweb proxy______Get______>Web server

2.User<_____200 Ok,Set CooKie__Fortiweb proxy<_______200 Ok, Set Cookie______Web Server

3.User________Post, Set Cookie_____> Fortiweb proxy_______Post, Set Cookie_______>Web Server

4. User________200 Ok,Set Cookie auth_____>Fortiweb proxy_____200 Ok, Set cookie Auth___________>Web Server

Now we got Alert_Deny as cookie Decryption error. I want to know which traffic follow will get alert_deny? line number 3 or 4. 

Who can explain me cookie session1 security mode encryption and signed with traffic flow diagram?