Hi Graham,
Thank you for your interest in Syncfusion products.
HTMLConverter internally makes use of a windows component which runs only in STA mode. This issue could be resolved by invoking the instance of HTMLConverter separately, under STA by using the following code snippet.
Thread t = new Thread(new ThreadStart(Convert));
t.SetApartmentState(ApartmentState.STA);
t.Start();
Please refer to the following sample and let me know if you have any questions.
I73948.zipRegards,
Geetha