Posts don't come through to Mattermost - 502 error

I’m trying to use the chat integration to send posts to my Discourse through to Mattermost, but unfortunately, nothing appears at the Mattermost end.

While the existence of the error is reported in the plugin settings, clicking on the icon display no further details, and nothing appears to be recorded in the error logs on the Discourse. Attempting to send a test message through produces a ''502 error" message.

i haven’t found another way to generate a 502 error from that same incoming webhook URL.

Any thoughts on where I might go from here? Thanks in advance.