OAuth 2.0 HTTPs Access Token Retrieval Page not displayed

Hi

SoapUI Version 5.2.1

I try to retrieve a Oauth 2.0 token from a server. When I run it over http, I see the Access Token Retrival login screen and I'm able to get myself a token in the SoapUI embeded browser.

But when I use https for retriving the token (as it should be in our stage environment), I get an error in the Browser:

"The page could not be loaded".

When I enter the same Authorization URI in firefox (or similar) I can open this page (=The URL is not wrong). The SSL Certificate on the Server is not trusted (But i can manually add it in firefox).

Could that be the problem? But how do I add this Certificate to SoapUI.

I already tried to add the certificate of the server with the java property:
-Djavax.net.ssl.trustStore=truststore-stage.jks

-Djavax.net.ssl.trustStorePassword=<truststore-password>

But it was no use.

Thanks for your help

Christoph