Hello, I have a similar problem:instead that transparent makes them black...
I looked for the folder with temporary files but I didn't find it!
I even tried with the autodesk hotfix
registry cleaning does not solve
Any ideas?
They are usually black for lack of light or texture.
Try to make a new file and put two walls with layers and some doors and then render and let me know.
If the file is old you could see everything black.
I believe I have solved: in the uninstall procedure indicated by the autodesk site he said to empty the folder indicated by frc after the disintallation.
to reach it just write in the address bar %temp% !!!
Thank you all!