$ sftp lnussel@somehost.suse.de
Connected to lnussel@somehost.suse.de.
sftp> ^D

$ csync /home/ln/Desktop/shared
sftp://lnussel@somehost.suse.de:/home/lnussel/Desktop/shared
csync_sftp - the host key for this server was not found but an other type of
key exists.
csync_sftp - an attacker might change the default server key to confuse your
client into thinking the key does not exist
The host key for the server tanana.suse.de has changed.
This could either mean that DNS SPOOFING is happening or the IP address for the
host and its host key have changed at the same time.
The fingerprint for the key sent by the remote host is:
: 3c:03:e6:15:67:b3:1d:0a:8f:86:15:95:60:49:c0:da
Please contact your system administrator.

[2019/03/07 13:50:25.572020, 1] csync_timediff:  Access dienied to remote uri:
sftp://lnussel@tanana.suse.de:/suse/lnussel/Desktop/shared - 
[2019/03/07 13:50:25.572095, 1] csync_init:  Synchronisation is not possible!
csync_init: Operation now in progress

also tried ssh -4 and ssh -6 explicitly. both work without complaints


Except for a build fix csync didn't changed compared to 15.0 where this still
worked. libssh was version updated from 0.7.5 to 0.8.4 though.