The documentation is out of date, you don't need it in .htconfig.php anymore, but it shouldn't have locked you out of admin either.

Have you put it near the top and missed a semi-colon?

$a->config['facebook']['appid'] = 'xxxxxxxxxxx';
$a->config['facebook']['appsecret'] = 'xxxxxxxxxxxxxxx';

You can try adding any missing punctuation, or deleting those lines entirely, and entering your keys on the plugin page instead.

There's also quite a lot of strange cache issues with Friendica. If you're using fast cgi, try restarting it. Also try restarting your browsers (especially if it's Firefox) instead of just clearing the cache.

Yeah, now I read that I didn't need to edit the htconfig, but when I just delete these two lines (which were at the very end) I just can't use the facebook connector anymore ("missing API key") but I'm not able to get to the plugin settings, because I can't get to my admin settings xD

Restarted browser (both iron and firefox tried), didn't help.
Honestly, I don't know how to restart fast cgi (or if I'm using it xD) , I'm hosting on uberspace.de

If you don't know whether your using fast cgi, you either aren't or can't influence it... one of those two alternatives is indeed the case on shared hosting with Uberspace. So you can forget that aspect.

Have you changed the file permssions or owner when you edited the file, maybe?

It has to be readable by the webserver.

Also check the line

$a->config['admin_email'] = 'YourEmail@Domain.com';

make sure you haven't knocked another key while you were scrolling down.

Oh shit, what a stupid mistake!! :D
I changed the admin-email, because I thought this would only be the place where my mails go. I didn't really think about it and didn't even remember when posting this...

Shame on me :D
And thank you so much for your fast help!