Hi @hartiberlin,

Please Note:
Forum to Forum migration is a large project. We’re sorry but we cannot support issues arisen during your forum migration. We only support questions related how to use and configure this tool in wpForo support forum. We don’t support the result of migration. This kind of large work is out of our support. In case the migration result of this tool doesn’t satisfy you, you should contact to professional forum migration services for custom migration. We only recommend gConverter or ProfProjects services for such projects. They have already done hundreds of forum to wpForo migration projects with the best result.

Also mention that Go2wpForo tool is used to transfer the basic data of your website, not for the attachments, private messages etc. I recommend gConverter or ProfProjects services for migrating large forums.

1. About Attachments:
For migrating the attachments properly, You should install and activate wpForo Advanced Attachments addon before the migration.

2. About YouTube embedding:
If you need only YouTube embedding, Just put this code in active WordPress theme's functions.php:

add_filter('wpforo_content_after', 'wpforo_custom_video_embed', 10);
function wpforo_custom_video_embed( $content ){
    $paterns = array();
    $paterns[] = "/<a[^><]+>\s*[a-zA-Z\/\/:\.]*youtube.com\/watch\?v=([a-zA-Z0-9\-_]+)([a-zA-Z0-9\/\*\-\_\?\&\;\%\=\.]*)/i";
    $paterns[] = "/<a[^><]+>\s*[a-zA-Z\/\/:\.]*youtu.be\/([a-zA-Z0-9\-_]+)([a-zA-Z0-9\/\*\-\_\?\&\;\%\=\.]*)/i";
    $content = preg_replace($paterns, "<iframe width=\"420\" height=\"280\" src=\"//www.youtube.com/embed/$1\" frameborder=\"0\" allowfullscreen></iframe>", $content);
    return $content;
}

Also, check out this paid addon:
https://gvectors.com/product/wpforo-embeds/

3. About installing the addons before or after migration.

You should install them after the migration, except the Advanced Attachment addon which is needed for migrating the attachments you have.