If you are getting "mixed content" warning, please check if all rows in the wp_options table use the correct https:// URL. You can quickly find the offending rows with this SQL query:
select * from wp_options where option_value like 'http:%';
Once you've ensured that there aren't any http:// URL left, please click "Clear Cache" from the VamTam menu in the admin bar (or click "Save Changes" in VamTam/General). This will regenerate the cache/all.css file.
Q: I installed the Really Simple SSL plugin, but still there are stocks in the CSS which have the http:// adress instead of https://. How can I resolve it?
A: In general, there aren't hardcoded URL-s. You simply need to set the Settings / General / Site Address option up.
However, it's possible that Beaver builder caches some http:// URL-s of dynamic content, like pages, posts, etc, added before start using the https:// URL-s. If this is the case, you have to update the URL-s in the database, using the wp-cli or this plugin https://wordpress.org/plugins/better-search-replace/. You have yo clear cache from VamTam/Clear Cache and VamTam Builder/Settings/Tools/Clear Cache.