Today tried again, but this time running UBPorts Installer on Ubuntu 18.04, deb-version. Still have same result. UBPorts stdout says:

peter@peter-pg-lt-gps:~$ sudo ubports-installer 
Gtk-Message: 16:10:56.149: Failed to load module "canberra-gtk-module"
info: Welcome to the UBports Installer version 0.4.5-beta!
Pug interceptor registered successfully
info: Downloaded file 1 of 1
checked: keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz true
checked: version-5.tar.xz true
checked: device-b4422d1593b76acedd819149ddb24b505a9cc345c6d73cf8094996a926b44ca1.tar.xz false
checked: ubports-ab82f71538d423d2ede781431e719918bb0c3450f148c230407f58083c9b7e77.tar.xz true
checked: device-b4422d1593b76acedd819149ddb24b505a9cc345c6d73cf8094996a926b44ca1.tar.xz true
info: Good bye!

UI of installer still says 'Sending ...' endlessly, device stays at this 'reboot system now'/'ubuntu actions'/'android actions' menu. No reboots could help, nor to 'recovery', nor to 'fastboot'.

Any advice? Thanks!