"400 Bad Request" error after upgrading

Hello. I have upgraded our forum from v2.3.0.beta5 to latest and now the website gives a “400 Bad Request” upon reaching it.

If I rebuild the app, as soon as the container is stopped, server gives a “502 Bad Gateway” error. But after finishing the rebuild and starting the container, error 400 appears again.

I was not responsible for the forum for a while so the upgrade was done very late. The forum only has official plugins. I have disabled some of them and rebuild the app to no avail. Do you have any ideas about the cause and how to fix the problem? Thanks in advance.

Edit: Some warnings I got:

I, [2021-01-06T16:46:07.011565 #1]  INFO -- : > cd /var/www/discourse && find /var/www/discourse ! -user discourse -exec chown discourse {} \+
I, [2021-01-06T16:46:10.379601 #1]  INFO -- :
I, [2021-01-06T16:46:10.380578 #1]  INFO -- : > cd /var/www/discourse && su discourse -c 'bundle install --deployment --retry 3 --jobs 4 --verbose --without test development'
[DEPRECATED] The `--deployment` flag is deprecated because it relies on being remembered across bundler invocations, which bundler will no longer do in future versions. Instead please use `
bundle config set --local deployment 'true'`, and stop using this flag
[DEPRECATED] The `--without` flag is deprecated because it relies on being remembered across bundler invocations, which bundler will no longer do in future versions. Instead please use `bun
dle config set --local without 'test development'`, and stop using this flag

Post-install message from i18n:

HEADS UP! i18n 1.1 changed fallbacks to exclude default locale.
But that may break your application.

If you are upgrading your Rails application from an older version of Rails:

Please check your Rails app for 'config.i18n.fallbacks = true'.
If you're using I18n (>= 1.1.0) and Rails (< 5.2.2), this should be
'config.i18n.fallbacks = [I18n.default_locale]'.
If not, fallbacks will be broken in your app by I18n 1.1.x.

If you are starting a NEW Rails application, you can ignore this notice.

For more info see:
https://github.com/svenfuchs/i18n/releases/tag/v1.1.0

Just an update; the problem was fixed by modifying the http headers used when forwarding over reverse proxy (the English terms could be different, sorry if this sentence is confusing).