Update 15 has broken Civil 3D dwg compatibility

I just installed Update 15 and now it seems that I can no longer open Civil 3D dwg files that previously worked fine in Update 14.  Microstation spins and spins on it for a while, consuming more and more memory until all 32 GB of RAM on my computer has been consumed, at which time Microstation crashes, along with a few other applications that I had open.  I tried this on 3 different drawings with the same result.  All of these drawings open fine in Update 13 and 14.