To enable generation of a trace file for Identity Server 3, insert the following in web.config file. If <system.diagnostics> is not already present in the file, insert the section between the sections <appSettings> and <system.web>. To disable the trace later, it'll suffice to comment out the <add> element.
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The initializeData attribute in the <add> element points to the trace file. When no path is specified, the file is created in the web site folder which may be inappropriate and can be a security risk.
Consider a better place to store the log and be aware that the application pool identity (service account) must have write access to the this folder (which , in fact, it may not have to the web site folder).
More information about logging
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