If not, then you are probably using lowercase ticker symbol - like yhoo for which message board is broken.
Try using YHOO - all uppercase.
I see that it fails if lowercase ticker symbol is used in the URL, like this - finance.yahoo.com/mb/yhoo or lower case yhoo is typed in "Get Message Board for" box and hit go
If uppercase YHOO is used, things are working fine.
Is this the links -- http://info.yahoo.com/center/us/yahoo/ ?