Ralf,
check https://tracker.moodle.org/browse/MDL-43312
Changing core tinymce from version 3.5.8 to 3.5.10 isn't a huge operation.
You could also tell IE11 users to press F12 in IE and make IE11 emulate previous versions of IE if they have difficulties with any sites where IE11 is supposed to be detected as IE, not Mozilla ("like Gecko")