I´m sure it must be easy for you...
Not really, but thanks to another user I have installed VirtualBox on my W10 workstation and created an OMV4 and OMV5 Virtual machine, needless to say they set up with Raid5 arrays, but with only small virtual disks.
I cleand all the drives with geparted,
That might be the problem in Storage -> Disks wipe the drives you want to add to the array you can select short from the dialog box that comes up.
With drives if you want to use them in OMV best practise is to prepare them in OMV