After upgrading to v91.1. I also ran into that issue, but only for Windows machines that had Splunk Enterprise installed. The Linux installations were not affected. I fixed it by replacing the ...\Splunk\etc\apps\search\bin\sendemail.py with an older version. Now I am getting integrity check errors, but e-mail alerts work fine. There is another post that says this issue might be fixed in v9.1.2. Let's see.. https://community.splunk.com/t5/Splunk-Enterprise/What-is-happening-in-Splunk-Enterprise-V9-1-0-1/m-p/651297
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