Drupal 8 Mixed Content Error, Loaded site on https but resource (css js) over http (not secure)

i was having this problem on one or more of my sites page.
what is weird is, that the problem only exist on every “form” page. site/form/*

As you know on Drupal 8, i can just include my js and css files from Drupal UI, but everytime i did, it alwasy show this error

Mixed Content: The page at 'https://mysite/form/example-form' was loaded over HTTPS, but requested an insecure stylesheet 'http://www.staging-achilles.systems/id/webform/css/example-form?oze1e1'. This request has been blocked; the content must be served over HTTPS.

this message always shows everytime i edit the css and js on the Drupal Form UI.
I tried to find how to fix this, but lot of the guide is explained on Drupal 7, and when i put it on my code, it doesn’t work.

i am not really good at PHP so i don’t really understand how to tweak the code..

this is what i already tried

  [1]: https://www.drupal.org/node/2419337
  [2]: https://www.drupal.org/node/1670822

Note: I cant do anything on html level, like changing the path or else. i also couldn’t know where this kind of thing is configured, like, is it even possible to change the path myself? or etc..

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/4/feed