The problem is caused by an input without the attribute mode defined. This attribute is required, without it the input definition is invalid. Therefore, the migration cannot operate on it. You must either define the mode for this input (batch, rising, advanced) or remove it completely before running the migration.
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You may be having the same problem here:
https://answers.splunk.com/answers/518247/how-to-resolve-unable-to-initialize-modular-input.html?childToView=615146#answer-615146
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I had similar issue. I removed settings.py and other settings.* files from bin folder, restarted splunk and the situation was the same.
Then I reinstalled the app (=downloaded the app file and uploaded it over web interface with checked update checkbox), restarted Splunk and after restart everything was OK again.
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