I had this error on Splunk Enterprise on Linux, I modified the commands for Linux, restarted my server and all is well...
openssl x509 -enddate -noout -in server.pem
notAfter=May 29 17:13:31 2017 GMT
/opt/splunk/bin/splunk createssl server-cert -d "/opt/splunk/etc/auth" -n server -c YOUR.DOMAIN -l 2048
Create certificate server.pem signed by the root CA.
Store the server.pem key file locally with your client/server application.
Enter a secret passphrase when requested.
The passphrase is used to access server.pem in your application.
Enter the application's hostname as the Common Name when requested.
Enter the root CA passphrase (Getting CA Private Key) to sign the keyfile.
The keyfile will expire after one year or sooner if the root CA expires.
Generating a 2048 bit RSA private key
.+++
...................................................+++
writing new private key to 'serverkey.pem'
Signature ok
subject=/CN=YOUR.DOMAINt/O=SplunkUser
Getting CA Private Key
subject= /CN=YOUR.DOMAIN/O=SplunkUser
issuer= /C=US/ST=CA/L=San Francisco/O=Splunk/CN=SplunkCommonCA/emailAddress=support@splunk.com
notBefore=Mar 12 15:56:47 2018 GMT
notAfter=Mar 11 15:56:47 2021 GMT
service splunk restart
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