Transaction sso2 - The Digital Signature for This Certificate Cannot Be Verified

Hello Experts,

I am encountering an issue in transaction SSO2 which relates to importing a certificate from Enterprise Portal to ECC for the purpose of using Portal to generate Logon Tickets and the ABAP system to accept the Logon Tickets.

Process:

I have exported the SAPLogonTicketKeypair-cert.cert from our Enterprise Portal

I then logon to client 000 of our ECC system and run transaction STRUSTSSO2

I import the certificate, add to certificate list and add to ACL

I then run transaction SSO2 and execute it with blank fields and the following warning is displayed - "The Digital Signature for This Certificate Cannot Be Verified" (as per below image)

I perform the same actions using the same certificate on our other ECC and SRM systems and everything works fine, testing of logon tickets works fine too.

Would anyone have any ideas as to what I can do to try and resolve the issue.

I have searched SCN, Googled, etc... but not found anything of value.

Thanks in advance for any responses