"Missing required paramter" with all Social Auth providers
I've found the reason for the problem: outgoing proxy. The web server where I have my website installed doesn't have direct internet access, but it uses an outgoing proxy.

Regarding social_auth_twitter - it was my mistake - outgoing proxy was blocking access to api.twitter.com. After configuring the proxy to allow this traffic - Twitter authentication started working as expected.

But regarding the other two (social_auth_google and social_auth_linkedin) - no requests ever reached the proxy, meaning that these modules don't know how to use the proxy.

I've ensured that the proxy is configured correctly:


Environment variables http_proxy, https_proxy, no_proxy, HTTP_PROXY, HTTPS_PROXY, NO_PROXY are configured as needed;
In VirtualHost element (Apache configuration) I'm ensuring that these environment variables are available by setting the appropriate PassEnv lines (mod_env module);
settings.php file contains the appropriate $settings['http_client_config']['proxy']['http'] (and other) configuration.


At the end of the day - everything else (installing and updating modules, etc.) is working normally through the proxy, so proxy configuration isn't the problem. 

It looks that, at least for now, you cannot have social_auth_google and social_auth_linkedin if your web server is behind a proxy. Another very disappointing fact is that there are no any logs in /admin/reports/dblog, so you cannot know that modules are struggling to access their corresponding API endpoints. Hopefully it'll be resolved in future versions. For now you simply have to allow direct access, and remove outgoing proxy completely.