This seems to be related to this bug that was fixed awhile ago. .NET 5.0 - HtmlFragment - System.NullReferenceException
For some reason we only get this error if we nest html lists like <ul><ul></ul></ul>
We are running on Amazon Linux and I have set up everything in the FAQ
Instead of Aspose.PDF for .NET, can you please try using Aspose.Pdf.Drawing? You can simply uninstall existing Aspose.PDF for .NET from your project and install Aspose.Pdf.Drawing from NuGet Package Manager. This API will not need any code changes. Please try it and let us know if issue still persists. Please share sample code snippet with us as well that you are using. We will further proceed accordingly.