No idea if this has anything to do with this problem but check to see if you have Protected Mode turned On and if so turn it off and try to open the PDF again. You do that from Edit-Preferences-General, then in the Security (Enhanced) section there is a check box "Enable Enhanced Security"- uncheck it.
REPLY: OK, so I did as you suggested. I went to the enhanced security tab and unchecked the box as you described. If I go to an email now and try to download a .pdf attachment, it now tries with adobe reader as the default program, but I get the same error message. If I click on view the attachment, however, it opens it in google drive and I can see the opened file there.
I noticed something else. If I try to open Reader normally, I get the same error code.
But if I right click on the shortcut icon on the home page, and open reader as administrator, it runs fine.
What does that suggest to you, Dimitri?