Quite an interesting problem.
If you set encoding headers for XMLHttpRequests to header(‘Content-Type’ , ‘text/html; charset=utf8‘); instead of header(‘Content-Type’, ‘text/html; charset=UTF-8‘); IE7 will give the nice and clear error c00ce56e.
So if you ever get this problem, here’s the solution.
update:
Sorry for my brief description of the problem. The problem occurs when setting the charset of a response to a XMLHttpRequest to utf8 instead of UTF-8. IE7 is very strict on naming of the charsets, while Firefox and IE6 (!!!) are a bit more lenient.
I read Microsofts description of the problem at http://support.microsoft.com/kb/304625 where they describe it as “Behavior is by design”. 
I’ve run into this aswell actually. I dont even recall it giving any errors, just failing silently.. perhaps it was IE6.
It actually works in IE6! but as I wrote in my update, IE7 is much more strict on naming.
Hello, j’ai résolu le problème en enregistrant la page contenant le script ajax en utf-8 sans BOM depuis l’éditeur de code.
Le plus simple est de d’encoder toutes es pages en UTF-8 pour éviter ce genre d’ennuis qui font perdre du temps inutilement.
Merci
Amin
[...] Error c00ce56e in ie [...]
oh thanks, this saved me a lot of hours on debugging this stupid, intolerant IE
cheers marc
You just saved my life, i was working about 24 hour on that and haven’t found solution.
Thanks Thanks Thanks
I had a similar error:
Server returning:
Content-Type: text/html; charset=ISO 8859-1
=> c00ce56e in ie
Content-Type: text/html; charset=ISO-8859-1
=> works fine
[...] anders.tyckr.com is die ursache, dass ich (wie jeder normale mensch) in json.php ein header(‘Content-Type: [...]