I ran across what I think is the same problem and posted a separate question and answer for it here: https://answers.splunk.com/answers/513409/duo-splunk-connector-error-validation-for-schemedu.html
If you're running into the same issue we were this error occurs because the Mod Inputs validation on allows the validation to run for 3 seconds before forcibly terminating it. Workaround is to build the inputs.conf file manually
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