Found this in your error log ?
PHP Warning: Xdebug MUST be loaded as a Zend extension ...
You've probably installed xdebug.so or php_xdebug.dll using a line similar to this in php.ini.
Well the fix seems to be simple, as seen in xdebugs installation instructions.
You need to use a different directive, and you need to specify the entire path to the so/dll.
Making Windows something similar to this
zend_extension_ts = "c:/php/modules/php_xdebug.dll"
And making UNIX/PECL something like this
zend_extension = "/usr/lib/php5/modules/xdebug.so"
So far the non-_ts version is working on my Apache2 test box, though.
Update: Using "zend_extension_ts" didn't work at all on my Debian/Apache2 box.
Now if you're using the new directive and still getting errors, which happen to come half-a-dozen at a time, in your logs, then you will want to add the directory used for extension_dir to the value of include_path in php.ini.
; Assuming the following
;extension_dir = "/path/to/extensions"
;extension_dir = "c:\path\to\extensions"
; UNIX: "/path1:/path2"
;include_path = ".:/usr/share/php:/path/to/extensions"
; Windows: "\path1;\path2"
;include_path = ".;c:\php\includes;c:\path\to\extensions"