The old Kubuntu distro did not start anymore (or only in emergency mode) Dual boot partitions were taken over manually as they were, leaving bootloader unchanged on /dev/sda during installation. Then comes the message:
No EFI System Partition was found. This system will likely not be able to boot successfully, and the installation process may fail. Please go back and add an EFI System Partition, or continue at your own risk.
What does that mean? It was started in EFI mode, which was pointed out by the GRUB menu at startup, and ..
[ -d /sys/firmware/efi ] && echo UEFI || echo BIOS
UEFI
New partition must be created especially for EFI GRUB 2/Basics (section "With EFI"). Can I do this in the installer or better in advance with GParted, can I uncheck the / or /home somewhere?