I read in the Debian Wiki (https://wiki.debian.org/UEFI#RAID_for_the_EFI_System_Partition):
"But for software RAID systems there is currently no support for putting the ESP on two separate disks in RAID." I interpret that as "it cannot work", so, the Debian Installer should refuse to install EFI on a Software RAID with an appropriate message, instead of running into this error.
