Hi,
I had the same problem on my HP Microserver Gen8 running OMV 4.
It turned out that I recently replaced the SSD on which the system was installed for a bigger one, and one of the HDD for a bigger one as well.
I had 2 issues causing slow boot and the "mdadm no arrays found":
- the /etc/initramfs-tools/conf.d/resume file was referencing the swap partition on the previous SSD (thanks @gderf)
- the replaced HDD was still found in /etc/fstab file
After correcting these 2 mistakes, no more mdadm messages on boot.