Hi Siddhesh,
As I don't see "self signed certificate" in the trace I would lean towards the fact that on the Websphere side the certificate chain is incomplete or the CA attributes are incomplete.
I would look at the Websphere truststore/keystore.
At least we have proved that the SAP side is not the problem at the moment. :-)
I still have the battle scars and grey hair when from we had these type of issues for NFE and Mexican Digitally signed invoices :-(
Cheers,
A.