when tested update 7U15 that the false "Expired certificate" warning is back with testing Webstart
out.

Also those message will be displayed not only as warming dialog but sometimes displayed as information dialog. Not sure when this discrepancy started.

In the OCIE picture, you will notice that the certificate shows as expired on the first dialog, but not
the second. The last dialog show that it is valid until 2016. (The Verisign certificates are also valid.)
Also note that there are 3 items:
1) A 'warning' that is wants unrestricted access
2) A 'warning' that the JNLP is not signed
3) An 'info' message showing that the certificate is trusted

Now look at the SignTest picture. This is the same certificate, but it does not show as expired.
Also, notice that there are only 2 messages in the second dialog.
1) An 'info' message about unrestricted access (NOT a 'warning' as in the first)
2) An 'info' message showing that the certificate is trusted

 
What makes these warning dialog so different?

An updated certificate was obtained recently and should not be expired.