For me, this was caused because of the Nginx server configruation. I originally had:
server {
listen 80;
server_name 12.168.101.224 mydomain.com www.mydomain.com;
# the rest of the config ...
. . .
And I fixed the error by putting my domain name first rather than the server IP:
server {
listen 80;
server_name mydomain.com www.mydomain.com 12.168.101.224;
# the rest of the config ...
. . .
Having the IP listed first confused Sendy's installer. There was no indication as to why Sendy didn't like my domain.