"Missing host" for 1&1 sftp access. Address is correct and ssh connects but sshfs does not

Logging into a 1&1 SFTP with sshfs-fuse does not work.

This is what I get.

tiff@Alexa-n-Daphne-Laptop-Ubuntu:~$ sshfs [redacted-user-id]@home721810660.1and1-data.host/ /mnt/1and1
missing host
see `sshfs -h' for usage

Uh, this does work with plain ssh.

tiff@Alexa-n-Daphne-Laptop-Ubuntu:~$ ssh [redacted-user-id]@home721810660.1and1-data.host
[redacted-user-id]@home721810660.1and1-data.host's password:
X11 forwarding request failed on channel 0

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
(uiserver):[redacted-user-id]:~$

So... why is that sshfs saying "missing host" while ssh can log in just fine?

Is this a bug or something?

Ubuntu release: 18.04 (bionic) amd64
sshfs version: 2.8-1 (sshfs-fuse not located, only "sshfs" package available)

What I expected to happen: a nice mount
What I got instead: "missing host". I already contacted 1&1 and they told me to contact the software manufacturer. This is an Ubuntu problem. Not a 1&1 problem.