Given the uptick in Mac OS users posting remote access problems I can only suspect a change on the Apple front. To find out if you have CERT issues, post your logs.
- Go to Settings > Manage > Troubleshooting in Plex Web App
- Click the Download Logs button
- Save the resulting zip file
You can now share the zip file to help investigate an issue you’re experiencing (for instance, by attaching the zip file to a forum post you make about the issue).