Most certificate errors are due to clock problems, i.e. your phone is showing incorrect time, the result is that the Whatsapp server will present a certificate, which from your phone's pespective, was issued in the future (which is impossible) and therefore it throws up an error and terminates the connection.
I do not have a Nokia Asha phone but I assume it's an S40 device which means it is capable of running Whatsapp. Never heard of people having to install certificates first and in any case I think Whatsapp certificates are issued by a Universally recognized CA.
Probable solution: Manually correct the phone's time, restart it and try using Whatsapp again. It should work.