Hi Guys,
I already fix the error I encountered by doing below procedure.
* Uninstall the GP Web Client
* Delete the database GPWEBCLIENTSESSIONCENTRAL & GPWEBMANAGEMENT in the SQL Server
* Reinstall the GP Web Client then assign the Externally Signed Security Certificate
* Then the GP Web client works fine.
In my case on the first day I install the GP Web Client I used Self signed certificate, when our client purchased a Externally Signed Security Certificate I just Uninstalled the GP Web Client (without deleting the databases of Web Client) then reinstall then assign the Externally Signed Security Certificate.. Hope this can help to others.
Thanks to all for having time to help on my issue.
Thanks,
Ronie