Yes, I have an account that has certain AutoCAD files that will not render. I believe it is something to do with all the external referneces they have in those files that have not been cleaned up before posting it. However, I have never been able to narrow it down to a more exact reason.
For them the solution is to modify the C:\program files (x86)\common\docuware\imaging\DocuWare.Imaging.Worker.config file and change the rendering engine for the DWG file type as follows.
<add filetype="AutoCad" extensions="dwg, dxf, dwf" decoders="Stelleent, AnyDWG"/>
This allows them to be viewed by makes the image less clear and hard to zoom. However, the file can still be downloaded and viewed in AutoCAD with regular clarity.