I had the same problem with IE7. I tried everything. Then I renamed the folder
wp-includes/js/tinymce/plugins/spellchecker
to something else. Just to make sure, the problem is not with the spell checker. Well, IE7 worked fine!
But just to make things weird – after this I deleted the whole tinymce -folder, and reinstalled it from the 2.3 packet – and everything still works, now with the spell checker?
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This problem has to be related to the spell checker. I think the problem is with the languages. It tries to search for wp′s local language – and if its not found, it should use English. I red from somewhere, it tries to use the translation from Google…
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Anyway, everything works for me now, but feels a little insecure.