Bad Binary Signature (HRESULT 0x80131192)

I have an intermittent issue with a vendors software that uses Winnovative Html to PDF converter. Randomly we will receive an error when a user kicks of SavePDFFromHtmlStringToFile method that returns a "Bad Binary Signature" error and then that App Pool will not "attach" the pdf any longer. If I restart the App Pool then it begins working again. My vendor is at a loss and the devs at Winnovative have never seen the error. I've looked for a consistent patter in the EventViewer and have not found anything in particular that proceeds it. Therefore my question is what could cause that error to happen intermittently and what kind of things should I try?

I've already tried:

• Replace the DLL with a known good version
• Making sure that it's compiled for AnyCPU
• Verified aspnet_merge