A SparesMissing event had been detected on md device /dev/md0

The cause was an erroneous spares=1 option in the mdadm.conf:

# definitions of existing MD arrays
ARRAY /dev/md0 UUID=621d5f15:cce75825:60273c48:78a7dac7
   spares=1

I'm not sure how this ended up there, but I suppose it happened when a device failed and was replaced.

Removing the spares=1 option or just recreating the mdadm.conf from scratch fixes the problem:

/usr/share/mdadm/mkconf > /etc/mdadm/mdadm.conf

I got the message, however I have a spare disk. The answer above puzzled me a little. The resolution appeared to be simple.

There was spares=2 in my mdadm.conf. The odd thing is - I haven't configured that. It must have been done by some installation script. I have two RAIDs with one spare each. The first one got properly "spares=1" and the second one spares=2.