I tried to install a jail with "bsdinstall jail /zroot/jails/jailname". I chose a mirror and the installer wasn't able to download due to false configuration on my pf firewall.
After I fixed the firewall problem the installer still wasn't able to download the sources and said "Invalid URL scheme" (No matter which mirror).
I investigated the destination folder and saw that there was a valid MANIFEST file but no downloaded sources.
After deleting the MAǸIFEST the installer was abel to download the sources and install my jail.
Addition: I guess the installer started downloading (where he possibly got the MANIFEST) but failed to continue due to active/passiv ftp file transfer (had restriced outbound acces configured)