I'am using WP 2.6 with german language settings.
In NextGen Gallery 0.98 the german language file for the tinymce3 plugin is missing or named wrong. This is causing 404 errors only visible in your logfiles.
"GET /wp-content/plugins/nextgen-gallery/tinymce3/langs/de.js?ver=311 HTTP/1.1" 404 6715
Copy de_de.js to de.js and edit the file. Change the String "de_de" in line 3 to "de". Now TinyMCE3 can find the language file.
Side note: "de_de" should be "de_DE" to be 100% correct. But I don't know if tinymce is smart enough to find "de_de".