I tried to configure a private registry using the host url I docker log'ed in to - which was like
https://my.host.name/v2/
It turns out that portainer didn't like that, but gave no immediate feedback. Instead it said:
"Image from container: invalid reference format"
when I tried to add a new container.
I recommend that when you add a registry portainer tries to validate it. If that's not possible then it'd be nice for the failure message to be way more explicit. Maybe including some technical error information.