@t-schuurmans Looking at the error message I can tell you that /dev/vda is the wrong answer in regards to a software raid array.
There was just another thread in the forum where someone else was doing the exact thing as you.
Edit: https://forums.fogproject.org/topic/17626/issues-with-capturing-an-image-with-a-raid0-array
I would suggest the same thing here. Set the global flag for mdraid=true
then schedule a debug capture. At the fos linux command prompt key in lsblk
and then see if fos linux assembled the array for you. If yes, find out the correct device name it is for the array. And finally back in the fog web ui, in the host definition set the hard drive to use the same name as what was discovered in the debug console.