I think that for some reason, you may have a partial definition for the "summary_forwarders" index in %SPLUNK_HOME%\etc\apps\sos\local . This index is not related to the S.o.S app. To the best of my knowledge, it is part of the Deployment Monitor app, which I assume you had installed at one point in the past.
If you don't have that app anymore, my recommendation would be to remove that index definition from the file in question, or to remove %SPLUNK_HOME%\etc\apps\sos\local\indexes.conf altogether if that is the only thing that it contains.
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