Resolution:
TWM supports the use of JCEKS and JKS keystore format. Other commonly used formats, such as PKCS12 (.p12), and a combination of private key with the associated public certificate are not supported.

Always use the keytool command line utility within JDK to generate the keypair. OpenSSL does not support creating keystore using a format that is proprietary to Java. It is mandatory that in gateway-config.xml, the "type" of the keystore must be specified and conforming to the actual format, either JKS or JCEKS.