This may be due to ServiceNow implementing TLSv1.2 and requiring SNI for connections to the server.
Try the following:
/bin/splunk cmd openssl s_client -connect yoursnowurl.service-now.com:443
If you receive an SSL handshake error, your ServiceNow instance has likely implemented TLSv1.2. You should also check the ServiceNow app's logs for "SSLEOFError: EOF occurred in violation of protocol" errors.
Next, run the following command. If it connects successfully, TLSv1.2 and SNI have been implemented.
/bin/splunk cmd openssl s_client -connect yoursnowurl.service-now.com:443 -servername yoursnowurl.service-now.com
The current version of the Splunk Add-on for ServiceNow app has an older version of the httplib2 library that does not work with SNI. Go to https://github.com/httplib2/httplib2/tree/master/python2 and download a copy of the httplib2 folder. Backup the httplib2 folder in the bin folder of the ServiceNow app and replace it with the version you downloaded. Try adding your ServiceNow account through the Splunk web interface again.
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