vCD virtual machine console is not accessible from OnApp side.
The most common issue here is where the console shows "Connected" but does not show anything or the flash message is permanently showing "Connecting..."
OnApp uses the same API call as the vCloud Director native UI, so it will commonly also not work directly in vCD as well.
OnApp 5.2.x (and later) with vCloud Director integration
You should check that the console proxy has been configured in System > Administrator > Public Addresses and ensure that the domain used is also publicly accessible without ACL restrictions.
This error usually occurs due to the console proxy being misconfigured within vCloud Director.