There’s a problem with the plex.direct DNS servers.
It is affecting remote access.
It is also affecting access via Plex Web using app.plex.tv. Pointing the browser at the IP address of the Plex server should work OK. http://server_ip_address:32400/web.
Thank you for the quick reply and information. The resolution just now resolved itself, thankfully. I find it interesting how different browsers handle a DNS issue like this.