[Typo3] BE tree frame problem

[Chris Paige]

I'm having a problem similar to this thread:

http://typo3.org/documentation/mailing-lists/english-main-list-archive/thread/37859/?tx_maillisttofaq_pi1%5Bsword%5D=alt_mod_frameset.php&tx_maillisttofaq_pi1%5Banswered_only%5D=0&tx_maillisttofaq_pi1%5Bmode%5D=1

The BE was working fine this morning. This evening, I can't get to the tree 
frame.  All of the BE links under Web and File result in a "resource cannot 
be displayed" HTTP 406 error ( An appropriate representation of 
/typo3/alt_mod_frameset.php could not be found on this server)

I've tried in IE, Firefox, and Opera (all Windows).  All the same.  Doc, 
User, Tools links seem to work fine.  It still works fine on another 
installation of Typo3 (different server), so I think it's probably not my 
computer.  I'm using Typo 3.7.0.

I wasn't doing anything fancy this morning.  Just working with content (text 
elements and tt_news).  Now I can't do anything...

Any ideas?

Chris