Windows could not start the Sitecore Xconnect Search Indexer - {xConnectInstance}-IndexWorker service on Local Computer. Error 1064: An exception occurred in the service when handling the control request.

I was doing upgrade from Sitecore 9 Initial Release to Sitecore 9 Update 1. I felt somehow confident that I have followed all the steps of upgrade guide correctly, but when I reached to the step in resuming service (page 47 in the document: “Start the Sitecore XConnect Search Indexer Windows service”), I faced the following error:

“Windows could not start the Sitecore Xconnect Search Indexer - {xConnect Instance}-IndexWorker service on Local Computer. Error 1064: An exception occurred in the service when handling the control request.”

Based on MSDN, this error means:

“While handling a control request, the service attempted to access an illegal address.”

I then tried to run this application manually from the folder containing it (C:\inetpub\wwwroot\<xConnectInstance>\App_data\jobs\continuous\IndexWorker).

It is clear that this error refers to the connection string which I obtained by copying and pasting directly (my mistake during installation) from the upgrade documentation, therefore, this issue can be simply fixed by modifying the “solrCore” connection in C:\inetpub\wwwroot\<xconnectInstance>\App_data\jobs\continuous\IndexWorker\App_config\ConnectionString.config to use “https” instead of “http” (Ensure that you have already setup the Solr with SSL correctly).

My index worker can now be started.

While there could be several other reasons for unable to start the service, I hope the steps above (such as to run the service manually and to see the exact error) will help to give some direction in fixing the similar problem too.