Opening CHM (Windows Help Files) downloaded from the internet or from a network location will most likely give you “Navigation to the webpage was canceled” and it will refuse to display the contents of the help file. This is due to changes that were introduced some years ago by a security update, and this is the default behavior in Windows Vista and 7 (and their Windows server equivalents).
I experienced this case at a customer site and to resolve the problem you can follow the steps in MS KB article KB896054. However adding the trusted UNC path caused some problems in this case, as the UNC path was sort of hidden because the CHM file was opened from an embedded link within an application.
So to find the UNC path from where the CHM file was opened I had two choices, either to use the famous Sysinternals tool Process Monitor to trace the open file action, but I chose a quicker way. Opened CHM files are logged in a file called hh.dat which can be found at C:\users\\AppData\Roaming\Microsoft\HTML Help. This file, when opened with WordPad contains the path to the opened CHM file, which lead me to the key which was an important part of this solution.