Please try loading the email using the following build of Rebex Secure Mail: https://www.rebex.net/getfile/be3a934cf8934a0fbb56b490ecd8e41b/RebexSecureMail-HotfixBuild7244-Trial-Binaries.zip
This should produce an exception with Base64-encoded RSA/PSS parameters of the signature - please copy&paste it here. It should make it possible to tell what went wrong.