When testing on your website, I can see that Flarum sends the wrong redirect url https://7onlineshop.com/auth/facebook
to Facebook, meaning the problem is on the Flarum side and not on Facebook.
Then we can see Flarum simply sends along the original request URI without any query string at https://github.com/flarum/auth-facebook/blob/f266439277a6b5412d777007894ff0619ef5cc1f/src/FacebookAuthController.php#L54
So this leaves only one option. Your Flarum is not receiving the full query path when decoding requests. Meaning the issue must be with the way your hosting is configured.
What kind of hosting configuration do you have ? Is it an actual subfolder, an aliased folder, a proxied folder ?
From what I observe I suppose you have some kind of proxy taking all requests to /flarum/*
and sending them to another virtual host or server that isn't aware of being a subfolder. For this to work it probably means in your config.php
has the Flarum url without /flarum
at the end ?