howto make "mainwp" work on linode?

Hi

I am running my websites on Linode (ubuntu) that I set up by myself.

I installed mainwp dashboard and mainwp child on the the same linode and when I try to add new site to mainwp I get

Invalid response from the server, please try again.
Hint: In case your dashboard and child sites are on the same server, please contact your host support and verify if your server allows loop-back connections.

• when installing dashboad - Dashboard System Requirements Checkup all passed

• I am using nginx

• I am using UFW

• I added these to my firewall

  > # Allow unlimited traffic on loopback

iptables -A INPUT -i lo -j ACCEPT

iptables -A OUTPUT -o lo -j ACCEPT

• I added new lines to /etc/hosts

127.0.0.1  localhost
127.0.1.1  ubuntu.members.linode.com  ubuntu
127.0.1.1  mymainwp.com mymainwp
127.0.1.1  child.mymainwp.cz child
xxx.162.xxx.xxx  mymainwp.com  mymainwp
xxx.162.xxx.xxx  child.mymainwp.cz  child

Could you please suggest what I can do to make mainwp work?

Thank you

Radek