There is much written elsewhere about problems viewing .chm files but I experienced an issue that doesn’t seem to be documented elsewhere so I thought I should share it.
I have a collection of .chm files I had copied to a folder called #Reference on my D: drive (the # was there to make it appear at the top of the list), but when I tried to view the content I got a message “This page can’t be displayed Make sure the web address //ieframe.dll/dnserrordiagoff.htm# is correct.”
I tried the other solutions for .chm files that are well reported, but none of them worked and strangely I found that if I copied the .chm file elsewhere I could view it without problem! After much digging I came across a reference to to this very old knowledge base article KB319247 which gave me the clue I needed. In my instance the # was in the folder name rather than the file name as mentioned in the KB, but it still applies. Once I removed the # the problem went away.
Note: I also found that using a % causes problems.