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Once the assemblies are registered to GAC, you can utilize the Syncfusion Conversion add-in without any issue. If the issue still persists even after the assemblies registered in GAC, please update your machine configuration (OS, Visual Studio along with updates) to reproduce the issue at our end.
Alternatively we can add the assemblies from installed location instead of GAC location using the assemblies from option before conversion.
Refer the following link for more information about Syncfusion Web Conversion add-in,