Well, I think that is weird :-)
This is the diff between autoptimizeHTML.php in 1.6.3 and 1.6.4;
futtta tags # diff 1.6.3/classes/autoptimizeHTML.php 1.6.4/classes/autoptimizeHTML.php
35c35
< if ($restore_mfuncs) {
---
> if (isset($restore_mfuncs)) {can you open the file and change it back to what it was in 1.6.3 to see if that solves anything (which would surprise me, really)?