Wallabag "Communication with server failed" not sure what I'm doing wrong here

[cactusboy]

So I finally got Wallabag up and running. I'm accessing it remotely via NGINX Proxy Manager and DuckDNS.

For some reason, KOReader on my PW5 seems to not be able to connect.

Here's what my settings look like on KOReader...

-- we can read Lua syntax here!
return {
    ["wallabag"] = {
        ["articles_per_sync"] = 30,
        ["auto_tags"] = "",
        ["client_id"] = "copiedfromwallabag",
        ["client_secret"] = "alsocopiedfromwallabag",
        ["directory"] = "/mnt/us/books/wallabag/",
        ["download_queue"] = {},
        ["filter_tag"] = "",
        ["ignore_tags"] = "",
        ["is_archiving_deleted"] = true,
        ["is_auto_delete"] = true,
        ["is_delete_finished"] = true,
        ["is_delete_read"] = false,
        ["is_sync_remote_delete"] = true,
        ["password"] = "mypassword",
        ["remove_finished_from_history"] = false,
        ["send_review_as_tags"] = false,
        ["server_url"] = "https://wallabag.myurl.duckdns.org",
        ["username"] = "wallabag",
    },
}

Obviously I've removed my actual password, client ID, secret, etc.

But the error I keep getting is this...

72.17.0.1 - - [27/Aug/2023:01:20:23 +0000] "POST /oauth/v2/token HTTP/1.1" 400 94 "-" "KOReader/2023.06.1 (https://koreader.rocks/) LuaSocket/3.0.0"

I gather that the 400 error means I'm giving it incorrect info somehow... but I can't figure out what I'm doing wrong here. Should it be HTTPS? HTTP? I've tried both. Trailing slash after the URL? Enable websockets on NGINX? Any other ideas?

[cactusboy]

Update. I decided to take NGINX out of the equation and changed my wallabag URL to just a LAN IP (192.168.1.233:PORT) and I'm still getting the same error 400.