I just had a read of the docs for fritz box. It appears it supports using webdav to connect to an online storage provider. I'm not sure it supports serving webdav.
The recommended way of connecting seems to be via smb/CIFS.
So unless you want to use smb/CIFS looks like we are back to FTP.
To work around this limit try setting this to 1. If that works then you can try higher numbers.
--ftp-concurrency int Maximum number of FTP simultaneous connections, 0 for unlimited