When I tried to upgrade my lab environment from vCenter 6.0 with external PSC to vCenter 5.5, I ran in to an annoying issue. I tried to upgrade my PSC, but the installer was not able to determine the version from my current PSC. It assumed it was 5.5 and I had to confirm this, which of course, I did not. No way to tell it it was really 6.0 …
After trying some stuff I found a log file which showed:
2016-11-15T20:47:38.547Z - error: vimService creation failed: Error: Service Unavailable (Failed to connect to endpoint: [N7Vmacore4Http20NamedPipeServiceSpecE:0x7fe24005a890] _serverNamespace = / _isRedirect = false _pipeName =/var/run/vmware/vpxd-webserver-pipe) 2016-11-15T20:47:38.549Z - error: Login failed for source VC (Not found for PSC): Error: Service Unavailable (Failed to connect to endpoint: [N7Vmacore4Http20NamedPipeServiceSpecE:0x7fe24005a890] _serverNamespace = / _isRedirect = false _pipeName =/var/run/vmware/vpxd-webserver-pipe)
But I was 100% sure I did use the correct root password for the PSC
Googeling did not show the exact cause for this problem, but since I had seen issues before with expired passwords on the VCSA, I logged in to the PSC with the root account, which did work … So I decided to change the root password …
After changing the root password, I was able to upgrade the PSC 
So most likely changing the root password “unexpired” the password. So the expiration for the linux password is not the same as the expiration for the password used for the API I presume …
By the way, if you would like to reuse the same password (by default you can not reuse a password that was already used before) you can issue the following command to clear the password history before changing the password:
echo “” > /etc/security/opasswd