"When checking for updates" - Where are you doing this?
You can try running "/etc/init.d/vsphere-client tail_log" on the H5client appliance and see if there's any errors there.
Your situation does sound weird though. Can you try pinging your PSC from the H5client appliance, maybe that's the issue?
I've got the h5client appliance set up with a windows vCenter 6.0 U1 server, installed it following the instructions, but it doesn't let me log in using the admin account I use for the regular webclient. Error: Unable to authenticate user. Please try again.
Logging in with the root account works, all settings like dns and time are ok. Any idea why it fails to log me on? Any logs I can check? First feedback tip: On the root config webpage, add a button to check the vcenter connection.
And something else, when checking for updates I got this error:
Failed to check for updates(Error validating the digital signature of the update manifest. Please contact your vendor.) on Thursday, 2016 April 07 11:22:26 UTC+2