just to wrap this issue up: as FMA didn't report any other error than
the one which led to the degradation of the one mirror, I detached the
original drive from the zpool which flagged the mirror vdev as ONLINE
(although there was still a cksum error count of 23 on the spare drive).
Afterwards I attached the formerly degraded drive again to the good
drive in that mirror and let the resilver finish, which didn't show any
errors at all. Finally I detached the former spare drive and re-added it
as a spare drive again.
Now, I will run a scrub once more to veryfy the zpool.
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