I have this issue with Scenegraph.dll installing ArcEngine on an ArcIMS machine. We need to install Engine because we are going to host Image Services. I get the following error:
'Error 1904. Module D:\Servers\ArcGIS\bin\SceneGraph.dll failed to register...
I found this related solution and did try the manual registration:
So the 'resolution' ended up being to just install the 9.21 upgrade which will run without error. Not ecstatic about that as a solution, but the best i can get.