This is no longer an issue. Is export fine with latest packages:
<PackageReference Include="DocumentFormat.OpenXml" Version="2.8.1" />
<PackageReference Include="Telerik.Reporting" Version="16.0.22.225" />
<PackageReference Include="Telerik.Reporting.OpenXmlRendering" Version="16.0.22.225" />
Problem was "xlxs" rather than xlsx".
If you have installed the Telerik.Reporting.OpenXmlRendering NuGet package, then you shouldn't need to also install the DocumentFormat.OpenXml NuGet package separately, maybe that is what is causing the issue?
Try to remove the DocumentFormat.OpenXml NuGet package from your project, and try to rebuild the project.
For example, the CSharp.Net6.HtmlIntegrationDemo references the following packages and is able to successfully export a report to XLSX:
<PackageReference Include="Telerik.Reporting.OpenXmlRendering" Version="16.0.22.225" /> <PackageReference Include="Telerik.Reporting.Services.AspNetCore" Version="16.0.22.225" /> <PackageReference Include="Telerik.WebReportDesigner.Services" Version="16.0.22.225" />