It turned out to be a firewall problem:
Port 5555/tcp (traditional Data Protector port) was blocked on the client.
As the installation is done using SSH (which was not blocked), downloading and installing of the software worked, but communication with the cell manager (CM) was not possible.
I used these CLI commands to configure the firewall (firewalld); you might want to adjust the port and the zone:
# firewall-cmd --permanent --new-service=omni
# firewall-cmd --permanent --service=omni --set-short="Data Protector"
# firewall-cmd --permanent --service=omni --set-description="Data Protector"
# firewall-cmd --permanent --service=omni --add-port 5555/tcp
# firewall-cmd --permanent --zone=internal --add-service=omni
# systemctl reload firewalld.service
Note that firewall-cmd allows to modify one item only (when trying to change multiple items, the rest is silently ignored (enhancement request exists already)).
Once it worked, it seems the inet process returns with somewhat random value what systemctl status omni* does not seem to like very much (it reports different errors).
I don't know whether those systemd unit files are in a good condition though (some additional tuning might silence those spurious errors).
I also found out that the installation still uses /etc/xinetd.d/omni if xinetd is installed (and running). However xinetd is not reloaded automatically, so it has to be done manually after installation to activate the omni service.
Finally Data Protector still installs some files with evil mode 0666; something one may want to fix (chmod go-w at least).
