I have tried multiple times to write out a very dense mesh combined with NURB surface data with Rhino 5. It appears to write the file out but when I re-read the file in, I get an error stating that some of the geometry is invalid. I finally found a solution which was to write the file out as a version 4 format, then it worked! I would like to send a file to you for evaluation. It's pretty big so I would have to send it via YouSendIt.
Please let me when I can do this.