Windows v2 players only download .htm files successfully from CMS 2.1.2

I’m running CMS 2.1.2 via Docker on Ubuntu 18.04, and a player on older Windows laptops (one Vista, one Windows 10 Home). This has worked for several years; it continued to work with both players when I switched from 1.7 to 2.1.1 CMS/Windows Player v2 this month.

However, when I updated the CMS to 2.1.2, neither of the two players can download the .xfl and .wmv files, but they are able to download the .htm (“text”) files. I can reach the admin page from Xibo Options, so the connection should be good. My Ubuntu machine’s firewall and my campus network firewall both allow access to ports 80, 443, 8080, and 9095 (perhaps unnecessarily on the network firewall).

On the player, I specify the URL to the CMS as https://192.168.1.200, which is the local IP for the server, so the network firewall should not be a factor. I followed the instructions in the Docker installation for providing the cms_custom-ports.yml file, so secure HTTP appears to be working via an Apache proxy.

I also followed a note to include RequestHeader set X-Forwarded-Proto "https" in the Apache proxy config file.

The error shown on the Player Info page (after turning on Info level logging) is Web Exception in Run: Unable to connect to the remote server and no resources other than .htm files show up.

I’m also getting a error on that same info page that the two layouts I have defined are invalid.

The requiredFiles document on the player seems to think the path to the resources is “https://172.0.0.1”. Is that correct?

I have reinstalled both the CMS and the client software, and even wiped the database to a pristine state but success still remains elusive.