Error Connection Refused

Okay, and now for the tricky part here, since your workstation is a Mac, and I haven’t got a Mac, so guessing here, but try this:

Open a terminal on a Mac, and type:

ssh admin@IP_OF_QNAP

If on Windows, use putty

Then type:

$(getcfg -f /etc/config/qpkg.conf PlexMediaServer Install_path)/plex.sh stop

Then type:

rm "$(getcfg -f /etc/config/qpkg.conf PlexMediaServer Install_path)/Library/Plex Media Server/plexmediaserver.pid"

Then type:

$(getcfg -f /etc/config/qpkg.conf PlexMediaServer Install_path)/plex.sh start

After above, paste a screenshot of the terminal window, as well as try and access Plex again

And STRONGLY suggest that you copy/paste the commands above into your terminal window to avoid type errors

And if SSH(Putty) Connection is refused, then make sure that SSH is enabled on your QNAP

/T