AEM 6.1 Forms - Set up certificates for Reader extensions: Error occurred while adding the private key

Hi,

I just tried to setup a Reader Extensions certificate in AEM 6.1 Forms and received an exception "Invalid keystore format". An error message on the screen says: "Error occurred while adding the private key"

I followed the description https://helpx.adobe.com/aem-forms/6-1/configuring-document-services.html. I did the same procedure some month ago in AEM 6.0 where everything worked fine. I tested with several certificate files, also with the one that worked in AEM6. In 6.1 they all lead to the same "Invalid keystore format"

Has anybody successfully imported a ares certificate into an AEM 6.1 user's keystore?

This is the exception:

Caused by: java.io.IOException: Invalid keystore format
    at sun.security.provider.JavaKeyStore.engineLoad(Unknown Source)
    at sun.security.provider.JavaKeyStore$JKS.engineLoad(Unknown Source)
    at java.security.KeyStore.load(Unknown Source)
    at com.adobe.granite.security.user.internal.servlets.KeyStoreManagingServlet.importKeyFromKeyStore(KeyStoreManagingServlet.java:479)
    at com.adobe.granite.security.user.internal.servlets.KeyStoreManagingServlet.doPost(KeyStoreManagingServlet.java:310)

Thanks for any help on this
Alex