I had the same issue and realized that the space on the server was less than 5GB (Default 5GB is the free disk space required for indexing to work). I have changed my drive to different ( to D from settings where I have 10GB) but it did not work ( did not try restarting splunk which I should have tried). I uninstalled (as I am trying the splunk on vm) and reinstalled on D and upload worked as expected.
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