Thank you for using Syncfusion products.
We suspect that the reported exception might be due to mismatched product version of Syncfusion assemblies. So, we request you to refer the same product version of Syncfusion assemblies to resolve this issue. Kindly refer the below KB link,If you have already installed the Syncfusion assemblies in your server machine, kindly remove those assemblies in your machine and GAC location, then install the required version assemblies to avoid version conflicts. Please refer the below links for removing the assemblies from GAC,
Can you please let us know which NuGet package, you are using for converting HTML to PDF, Kindly refer below KB link for converting HTML to PDF from NuGet package.KB: https://www.syncfusion.com/kb/10685/how-to-install-webkit-html-converter-using-nuget-package-manager
If still you are facing the same exception, we can have the web meeting to directly look into the issue on your server and provide the solution at the earliest. Please let us know your availability for web meeting, we will make every effort to have this scheduled on a time of your convenience.