501 Not Implemented Error on Menu Links

My website structure has a few level 3 items. When the user clicks on a left-column menu link from a level 3 page, the webserver gives this error message:

501 Not Implemented

The requested method is not implemented by the server.

The URL in the browser's command line looks correct, and when the user places the cursor into the browser command line and simply presses <ENTER>, then the requested page appears without complaint. (However, clicking on the browser <Refresh> button does not resolve the error message using the Chrome browser.)

This behavior is easiest seen with the News module: the list of News items is a level 2 page, and the news details are each on a level 3 page. The level 3 pages give the user access to the left-column menu items as well as to a "Go back" link at the end of the news story. The "Go back" link gets the user safely back to the level 2 News listing. But if the user clicks on the "News" link in the left-column menu (which should also go to the level 2 News listing), the "501 Not Implemented" error appears.

On other level 3 pages where there is no "Go back" link, the user is forced to use the browser's <Back> button to return to the level 2 parent page before having functional access to the left-column menu links.

How can this limitation be overcome?

To give readers a first-hand experience with this situation, please go to

http://www.josbacher.net/Lianne/inde...pionships.html

Then click on any menu item on the left, or click on a breadcrumb link. Do you get an empty page? (The "Go Back" link simply goes to your previous web page, so it only works when navigating on our website.)

This "501 Not Implemented" error also occurs when you are in a Video sub-subpage. Follow menu sequence Videos > 2011-2012 Season and then click on a left-menu item again. (The only recourse is the browser <Back> button.)