You are the best, Gustavo!
We did really have a condition rule in the .htaccess file which allowed the hotlinking only for the primary domain. We had to insert a row for the second domain as well
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{REQUEST_URI} ^(.*)?wp-comments-post\.php(.*)$
RewriteCond %{HTTP_REFERER} !^http(s)?://bayernszektor\.hu [NC,OR]
RewriteCond %{HTTP_REFERER} !^http(s)?://pannonbajor\.hu [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^$
Actually, we had to do this not only in the root folder but the wp-uploads folders since that also contained a .htaccess file with the same restriction.
Again, thank you very much for the amazing support (super quick and effective)