Hello Louwin,
We have seen this behavior when the motherboard does not have UEFI. Would you please check on BIOS is the motherboard comes with UEFI?
UEFI is needed when your hard drive or your partition is larger than 2 TB.
If you find this feature on BIOS, you would have to enable it, but it will be necessary to reinstall the operating system and recreated the RAID.
In your post, you mention that you are not able to boot to the operating system? Was the OS installed on that RAID?