This looks a lot like the bug described in LU-4460. Is there a fix/workaround for this in a current maintenance release?
This is exactly what is described in ticket LU-4460 I reported and
was just recently fixed.
Fix versions: Lustre 2.6.0, Lustre 2.5.1
AFAIK 2.4 is no more supported, 2.4.2 was the last one.
You should either rebuild 2.4.2 with this patch, or upgrade to 2.5.1
when it will out (end of march).
2.5 is the current maintenance release and it is fixed in the new
one.
As a workaround, as your are updating from 2.1.4, you just need to
change the mgsnid of your target to be ok.
failover NID are only used when building MGS config file. But, as
you are upgrading, those files are already present on MGS.
So, I advice you to use tunefs.lustre to only keep one NID for your
MGS and wait for a Lustre version with this fix to re-enable it.