Dear Haik,
Check the property 'ABAP Security Sessions Enabled' to 'Yes' in the portal system object, Please check the Note1471069.
Note: if you are not using HTTP Security Session Management on the backend, this value must be set to 'No'.
Please navigate to :
System Administration > System Configuration > System Landscape > Open System Object properties
Also check the service in SICF : /sap/public/bc/icf/logoff
Thanks & Regards,
Meghal Shah