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The article provides troubleshooting for commonly met issues with the HTML5 Viewer and its MVC and WebForms wrappers.
The recommended troubleshooting approach for the HTML5 Viewer is to use Fiddler or other proxy tool to check the requests, their responses and statuses. With Fiddler, information about requests and responses content can be seen in Fiddler - Inspectors - Request/Response - Raw tabs. This will let you check requests to the Reporting REST service and if the URLs are correct, if relative paths are resolved correctly. Relative paths may need adjustment depending on how the application is hosted - ASP.NET Web Project Paths.
The HTML5 Viewer displays content rendered through the Reporting HTML Rendering mechanism.
Error loading the report viewer's templates. (Template = /api/reports/resources/templates/telerikReportViewerTemplate-html).
The error message indicates that the Reporting REST service is not responding. Check the HTML5 Viewer's serviceUrl and adjust the relative path. Test adding "~" or remove the first "/" - ASP.NET Web Project Paths. It is recommended to test the service by address in browser - step 8 of How To: Add Telerik Reporting REST Web API to Web Application. The issue might be related to WebAPI controllers duplicating routes, which can be avoided by changing the registered by default Telerik Reporting REST Web API routes.
The HTML5 Viewer sends information about the client's reportSource.parameters in a JSON-formatted string. Test updating JSON.NET NuGet package to the latest official version through the Visual Studio NuGet Package Manager Dialog. Also verify that the HTML5 Viewer's persisSession is set to false.
The error message indicates that the viewer's ID cannot be found in the cache storage. Such error can occur if the time set as Reporting REST service's ClientSessionTimeout has expired - Cache Management, or if the Reporting REST service's Storage is not set accordingly the hosting enviroment - REST Service Storage.
The error message indicates that the Reporting REST service cannot access the configured file storage. Verify the Reporting REST service has an unique HostAppId in its configuration and the application has read/write access to the folder specified by the FileStorage instance in the Reporting REST service configuration - REST Service Storage.
The HTML5 Report Viewer does not display a report that uses Table/List/Crosstab item in Internet Explorer.
In case data visualized by the Table/List/Crosstab item results in a great number of HTML elements, Internet Explorer may fail to visualize the content. solutions are suggested in Design Considerations for HTML Rendering (Browsers Limitations).
The HTML5 Viewer is an HTML/CSS/JS widget, which gets content produced on the server by the Reporting REST service. Data objects and report instances cannot be submitted to the server in a request. Details how to use an InstanceReportSource with the HTML5 Viewer are avaialble in the How to display a report via InstanceReportSource in the HTML5 Viewer KB article.