I have what appears to be the same issue, except I encounter the issue when I try to access a file on my local drive in a folder that is a Symbolic Directory Link.  Without making any changes to ARX or anything else, I can get ARX to open the file simply by accessing the file using its actual folder path.  Maybe this will help...

C:\Folder1\MyPDF.pdf

• Result: No probblem.  This is the actual folder, and ARX can open the "MyPDF.pdf" file without issue.

C:\SymLink\MyPDF.pdf

• The "SymLink" folder is a symbolic directory link to the "C:\Folder1" folder. There is only 1 MyPDF.pdf file. Both paths reference the same file.
• Result: ACCESS DENIED. This error is using the Adobe "Cannot open... access denied" message, not a file system or Windows error message.

NOTE - Unchecking "Enable Protected Mode at startup" does prevent this problem! (Edit -> Preferences, bottom of "General" section). However, if you want the added security offered by protected mode, then you will need to avoid the use of symbolic links until Adobe addresses this issue. I am using version 10.1.4, the latest released version as of 10-06-2012 (October 6, 2012).

I have recreated this in multiple ways, with many different types of PDF documents, which were created by three different applications.  I have already spent too much time researching this, and found my workaround for this until it is addressed by Adobe.  In every case, it breaks ONLY when I try to access the file using a symbolic link in the path name.  Note that Windows 7/8 uses symbolic directory links in a few instances, and if someone uses those folders to store PDF's, they could have problems.

NOTE - Anyone can create a symbolic link in Windows 7/8 by using the "mklink.exe" command at a command prompt.  It is included with Windows.  I have not tested whether soft or hard links to the file itself will cause the problem, but I could upon request if anyone thinks it wouldn't be time wasted.  The time I can spend on a PC at one time is limited since a car accident, but I will do it if it could help other avoid the problem that I banged my head against a wall trying to figure out.

I hope this help others facing this issue, and I hope Adobe addresses this issue in an update very soon.

-TSForrest

Good luck and happy hunting!