that's probably du to the default cipher list being more strict with newer versions.
if so look at known issues : http://docs.splunk.com/Documentation/Splunk/latest/ReleaseNotes/KnownIssues
especially this one :
Older 6.0, 6.1, 6.2, 6.3 maintenance release versions unable to connect to 6.6.x and later via management port
Workaround:
This applies to License Master/Slave, Deployment Server/Client, Cluster Master/Peers, Search Head/Peers and affects Splunk 6.6.x and the following versions:
6.0.0 to 6.0.6
6.1.0 to 6.1.4
6.2.0 to 6.2.6
6.3.0 to 6.3.1
6.3.1511.1
Upgrade your older instances to the latest maintenance releases or on your 6.6.x Splunk instances. Add the following stanza to server.conf:
[sslConfig]
sslVersions = *,-ssl2
sslVersionsForClient = *,-ssl2
cipherSuite = TLSv1+HIGH:TLSv1.2+HIGH:@STRENGTH
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