A few days ago I've started testing grav, and when I've manually installed the admin plugin throw exceptions on plugins section or instead the admin plugin can't list local installed plugins and themes. I've started testing below a proxy configuration and later I went offline to test if that is the issue.
@rhukster suggest me to open an issue here to let the team know about it.. I hope that we will achieve an answer!!
The details of the issue are more detailed at the forum on:
http://getgrav.org/forum#!/getgrav/plugin-development:admin-plugin-dashboard-its
I am available for testing everything what you want!!