Which version of the AXG are you using? There are two known circumstances under which this particular error might occur:
1. The certificate does not have a X509v3 SubjectKeyIdentifier extension.
2. The encrypted message includes the entire certificate in the KeyInfo/SecurityTokenReference element.
If it's case 1, this bug has been fixed in the latest version of the AXG (5.1). Case 2 has been fixed but is not yet available in a released version.