It might be some kind of a bug, I recommended you try an installation with manual partitioning then.
Are you using UEFI? If so, it's the following.
First you'll have to use KDE partition manager to create 3 partitions:
/boot/esp/
(ESP - Efi System Parition) partition, with a size of 512MB and FAT32 as the filesystem, you'll need to set esp
and boot
flags on it. More information on it in the help center.
/
(root) partition with ext4 as the filesystem (your main system partition)
SWAP
partition with a size of 4-16GB (If you're planning to use hibernation, make it as big as your RAM, so 16GB)
After you make these 3 partitions, you can start the installer and choose the manual install option. On one of the next steps you'll need to select the appropriate partitions accordingly.
You could also try sharing the ESP and SWAP partitions between Solus and Kubuntu, but I'm not sure how well it would work. In this case you'll need to create only /
partition, and pick the already existing ESP
and SWAP
partitions (don't format the old ESP partition tho, because then Kubuntu will no longer boot).