If the file `WebReportDesignerSettings.json` doesn't exist before running the Web Designer REST Service, the latter will create it. However, the file will remain locked and the service will throw the exception:

"The process cannot access the file '/app/Telerik_Reporting/WebReportDesignerSettings.json' because it is being used by another process."

When the file exists, the problem will be avoided, which leads to the workaround - create the file at a starting point of the application. Here is a sample code:

static void InitFile()
{
            var path = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), "Telerik Reporting", "WebReportDesignerSettings.json");
            File.Create(path).Close();
}