I finally managed to install Ubuntu on my laptop.
The installer needed (lots of) help.
- I removed all partitions from the RAID (/dev/mapper/isw_dadfiaad_RAID0SYS2)
- I added GPT partition table to RAID (Device -> Create partition table)
- Then I created a small partition (about 350MB) at the beginning of the disk and added the boot flag to it (Partition -> Manage flags)
- I Created an ext4 partition (about 464GB)
- I used the remaining space (about 2GB) for swap
Error messages and warnings were poping up all the time, and gParted crashed quite a few times, but eventually it worked.
During the installation I partitioned the first partition as /boot/efi and the installation was successfully completed.