I hit the same error just recently. Here's what I did to fix it.
Exited OneNote. Ran task manager (right click in a free area of the taskbar and select "Task Manager") and ensured the process tab did not show OneNote.exe running uninstalled the addin looked in windows\assembly folder (which is probably hidden) and deleted the entry for OneNote Make Subpage reinstalled the addin made sure the dll was in the installed location (c:\program files\microsoft\make subpage folder) and copied it to the windows\assembly folder as well start OneNote
This worked for me. Since we cannot tell why this occurs, it's hard to find a fix for it. Installing our addins to both locations seems to lessen the occurences, though, and we are troubleshooting from that point of view.
