I must admit that I have no clue what to check next. Something in your main user account seems to interfere.
How about this: keep using the second user account. It doesn’t need to be an admin user.
Then disable the regular Autostart of Plex server in both user accounts.
Then use this tool PMS as a service to run Plex server as a Windows system service, under the test user account.
That way it is available as soon as the machine is switched on, even without a user logging in.
