Hi,
I've been experiencing this problem since I updated macOS to the latest version, Sierra. My mounted volumes (AFP shares from my home QNAP) are part of the Login Items of my account on my Mac Mini 2012. Everything worked great: autologin at startup, Volumes automounted, Plex Server starting and the then PlexConnect boot for my 3rd Generation AppleTV in the bedroom.
Now mounting volumes always require to insert credentials, already filled up in the pop-up window, but this step somehow breaks the startup of Plex Server and then PlexConnect. I experienced disconnections and moreover Plex Library on Apple TV is always empty as if it can't read from the server.
I need to dig down this issue but in the meantime any help for those who have a similar setup would be really appreciated.
Stefano