You are correct - anything that keeps WordPress from working is a bug in the plugin (most of the time :) ).
I put a patch into 3.3.14 which checks for Adsense Manager, and if it is enabled, it posts a notice to disable and quits.
It sounds like this did not happen for your installation - what version did you use?
The error you are receiving is that Advertising Manager cannot load any of the ad network files for some reason (e.g. all of the files in the advertising-manager/OX/Adnet directory). Did you put any additional files in this directory? Do you have any weird directory permissions?
Per your email to me, it sounds like Advertising Manager was working, and then stopped working. Did something happen with your ISP?
If you could give me as much detail as you can, I am happy to work through it with you.