Hello,
How about trying out nslookup cmd & ping to determine the culprit.i.e
nslookup clienta
once you know the client ip , do ping -a IP address ( reverse lookup) to 
make sure the resolution is returned with Clienta name, if not then you 
may have misconfigured client, dns or both.
Since legato error points to clientb and it's complaining that clientb 
needs to be alias of clienta, then do a ping -a on clientb's name along 
with nslookup,take this information to DNS admin and verify if they are 
accurate and adjust accordingly, you mentioned it being a DMZ client, in 
this case
there may be separate DNS server in DNZ that needs to resolve the names 
correctly.
HTH

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