Fatal error: Call to undefined method MoodleQuickForm_hidden::MoodleQuickForm_hidden() in moodle\lib\pear\HTML\QuickForm\element.php on line 363
We got this error message after upgrading to php 5.3. All the posts I could find said you had to either downgrade php or upgrade moodle. You can keep your installation of moodle and php by just modifiying some lib>forms files.
This website documents the changes needed "http://blogdeecomputo.blogspot.com/2011/01/moodle-call-to-undefined-method.html". It's in Spanish but the code changes are easily to figure out.
Just wanted to post this as it saved my bacon, and bacon is good.