In this case, a customer running Maya 2013 on Ubuntu reported this error when he tried to load the mtoa plugin:

API error detected in plugins/mtoa/Main.cpp at line 710: (kFailure): Unexpected Internal Failure 
ERROR | Failed to register renderer 'arnold' 

That specific line in plugins/mtoa/Main.cpp runs these two lines of Python:

import mtoa.cmds.registerArnoldRenderer
mtoa.cmds.registerArnoldRenderer.registerArnoldRenderer()

So I asked the customer to run that Python in the Maya script editor, and that gave us this error:

# Error: ImportError: file 
/usr/autodesk/maya2013-x64/lib/python26.zip/hashlib.py line 63: No module 
named _md5 # 

Now we’re cooking with EVIL gas! I’ve seen these kinds of errors before…

This appears to be general problem with Maya 2013 and pymel (I found a few different posts about this via Google search). For example, this thread from the pymel project, or the Troubleshooting section on this page.

In brief: you need to create some symlinks to the right versions of the libssl.