This was first mentioned by Patrick S. in the mailing list:
This is not a general problem and only occurs with original PDF generated with 3D content using Anark. The file when loaded seems to have encrypted and loads just find in Adobe Reader, but when we try to do a "Save As" we get the following error:
"The document could not be saved. There was a problem reading this document 21."
If I do a control click on the "ok" button. I get the following message:
"This direct object already has a container."
I can reproduce the effect with the attached file by using the Encrypt command line tool. A look at the file shows a double dictionary:
I don't know if this is the cause, but it doesn't belong there.