The error message states that the zpush site is not responding … it could be a temporary hiccup. To me, it seems odd that the address is a IPv6 address rather than IPv4.
Thinking out loud here … perhaps you could add an entry to /etc/hosts file adding the following line:
Hey, I feel like mentioning this to as many people as possible:
There is a working development build for Ubuntu 18.04 that it would help Mail-in-a-Box project if people would try upgrading to it on their non-business-critical boxes: