422 Unprocessable Entity when clicking on "Show full Post" in Discourse post

Hello,
I’m really hoping I can get some help here. Browsed the web, but couldn’t fine a case matching mine.

I’m testing Wordpress with Discourse. Separately they both work just fine. However I’ve got an issue when using the WP plugin.
When I publish a WP post, a topic gets created in Discourse using the plugin as desired. The topic itself is created correctly. However when I click the “Show full Post” button the WP post doesn’t get expanded.
In the browser console I see an error like this:

Request URL: http://forum.sadzinica.pl/posts/19/expand-embed
Request Method: GET
Status Code: 422 Unprocessable Entity
Remote Address: 192.168.0.15:80
Referrer Policy: strict-origin-when-cross-origin
Cache-Control: no-cache, no-store
Connection: keep-alive
Content-Type: application/json; charset=utf-8
Date: Tue, 12 Jan 2021 09:07:49 GMT
Referrer-Policy: strict-origin-when-cross-origin
Server: nginx/1.14.2
Set-Cookie: __profilin=REMOVED; path=/; HttpOnly; SameSite=Lax
Transfer-Encoding: chunked
X-Content-Type-Options: nosniff
X-Discourse-Route: posts/expand_embed
X-Discourse-Username: admin
X-Download-Options: noopen
X-Frame-Options: SAMEORIGIN
X-Permitted-Cross-Domain-Policies: none
X-Request-Id: b904c2d5-2088-4193-be0c-f36b497d716e
X-Runtime: 0.021514
X-XSS-Protection: 1; mode=block
Accept: application/json, text/javascript, */*; q=0.01
Accept-Encoding: gzip, deflate
Accept-Language: pl-PL,pl;q=0.9,en-GB;q=0.8,en;q=0.7,de-DE;q=0.6,de;q=0.5,en-US;q=0.4,it;q=0.3,es;q=0.2,ro;q=0.1,it-IT;q=0.1
Connection: keep-alive
Cookie: __profilin=REMOVED; _t=REMOVED; _forum_session=I-REMOVED-IT
Discourse-Logged-In: true
Discourse-Present: true
Discourse-Track-View: true
Host: forum.sadzinica.pl
Referer: http://forum.sadzinica.pl/t/zebranie-wiejskie-w-sprawie-x/15/3
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.111 Safari/537.36
X-CSRF-Token: 1ke5PaosmJ5gYeRpUs6ZkMWD+H4zaovMmVr3JPPNXeuidSmhdD2FmkeBaTgrH0TUuU+GrH/+OV0c338W/4RbjA==
X-Requested-With: XMLHttpRequest

And a jquery error:

XMLHttpRequest.send | @ | includes.js?v=10da95…f1bd50fe50ed714:819
  | send | @ | _ember_jquery-189e46…649161a18d.js:10113
  | ajax | @ | _ember_jquery-189e46…6649161a18d.js:9696
  | i | @ | _application-525b557…c2ebdfb8509.js:9229
  | (anonymous) | @ | _ember_jquery-189e46…649161a18d.js:72238
  | n | @ | _ember_jquery-189e46…649161a18d.js:72760
  | i | @ | _application-525b557…c2ebdfb8509.js:9242
  | expand | @ | _application-525b557…2ebdfb8509.js:16991
  | expandFirstPost | @ | _application-525b557…2ebdfb8509.js:78560
  | (anonymous) | @ | _application-525b557…2ebdfb8509.js:82798
  | value | @ | _application-525b557…2ebdfb8509.js:82756
  | value | @ | _application-525b557…2ebdfb8509.js:82794
  | click | @ | _application-525b557…2ebdfb8509.js:78431
  | (anonymous) | @ | _application-525b557…2ebdfb8509.js:75834
  | (anonymous) | @ | _application-525b557…2ebdfb8509.js:75733
  | value | @ | _application-525b557…2ebdfb8509.js:82756
  | k | @ | _application-525b557…2ebdfb8509.js:75732
  | (anonymous) | @ | _application-525b557…2ebdfb8509.js:75833
  | dispatch | @ | _ember_jquery-189e46…6649161a18d.js:5443
  | d.handle | @ | _ember_jquery-189e46…6649161a18d.js:5247

My setup: WP and DIscourse installed inside Ubuntu 20.04 LTS VM running on Virualbox under Linux. I’m testing withinmy local network with my own DNS directing to the host hosting the Virtualbox VM. I use 2 nginx reverse proxies. The first one in Ubuntu VM redirects requests to/from VM docker containers ports/unix sockets
Then this is forwared by Virtualbox to port 8000 on the host and then another nginx does reverse proxy to map this to port 80 of the host machine (host hosting the VM).
I’m testing this to simulate a real setup as I want to run both Wordpress and Discourse inside a VPS when I get everything working locally.

Please let me know if you need more info. Thank you!