Ok. From your suggestion I've replaced the html_header.php file with the 1.5.6c version, and that has gotten rid of the Mootools error, but I am now left with the following:

TypeError: el.addEvent is not a function. (In 'el.addEvent('focus', function(cal){ this.toggle(cal); }.pass(cal, this))', 'el.addEvent' is undefined) on line 454 of jscript_b-calendar.js

I've done a fair amount comparing of other files to try to find the problem, but have had no success.