vApp Manager for Solutions Enabler

Hello,

We are deploying the vApp Manager for Solutions Enabler (version 9.1) as an appliance on VMware ESXi.

However it fails to detect the Gatekeepers that were configured towards the ESXi hosts (version 5.5/6.5)

Within the vApp Manager, adding an ESXi hosts fails ("Failed to retrieve the issuer certificate")
And manually attached (physical RDM) gatekeepers to the appliance are not recognized or shown.

What are we missing here?
Is it allowed/supported to add manually gatekeepers to the appliance? 
And help or suggestions would be appreciated.

Cheers,
Harold