pretty weird John ...
The first things autoptimize.php does (on line 14) is including classes/autoptimizeConfig.php which loads the autoptimizeConfig-class. A bit later (on line 25 indeed), the configuration is loaded by calling the instance-method of the autoptimizeConfig-class. Without an autoptimizeConfig object, there can be no working Autoptimize.
Now, between 1.6.2 and 1.6.4 there was no change in the way autoptimize.php includes and instantiates autoptimizeConfig and classes/autoptimizeConfig.php did not change at all (I double-checked to make sure).
The only thing I can think of (however unlikely) is a problem installing the update, with (part of) classes/autoptimizeConfig.php gone missing or unreadable or ...
If you're up for it, I would love you to try to upgrade again (ideally disable and uninstall 1.6.2 first and only then install 1.6.4 from scratch). If the same problem occurs;
* check if classes/autoptimizeConfig.php is there (and is readable by webserver/ php/ wordpress)
* look at your php-error log or put WordPress in debug-mode and look at the output there.
* If nothing relevant can be found that way, I would try to downgrade to 1.6.3 to see if the problem was already in there (allowing us to narrow down the problem)
looking forward to finding & solving this nasty problem with your help :-)