@TWiseOne Just wanted to contribute to this discussion although it's bit old. I recently came up with the same error while working with the Splunk o365 add-on and the way I fixed it by editing the following file. /opt/splunk/etc/apps/splunk_ta_o365/bin/3rdparty/certify/cacert.pem I had to add the public key details of the Certificate used by the proxy server in it.
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