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If you check the mdadm detail output:
Raid Devices : 3
Apparently the software RAID in your configuration was created with 3 devices. 1 is missing and hence the RAID set is degraded. You will have to reduce the number of devices that are registered in the RAID set.
After failing and removing the drive, which you already did, I suggest the following:
mdadm /dev/md0 grow raid-devices=2 --backup-file=/boot/md0.backup
Since 2 disks are good enough for a RAID 1, --backup probably won't do anything.