Comments gets filtered out by ckeditor_filter_xss

Problem

The 'Teaser Break' button inserts a <!--break--> at the current location of the textfield. When the textfilter filter_html is active, then the function ckeditor_filter_xss() calls filter_xss() which removes any comments and therefore also removes the <!--break-->.

Workaround

Disable the textfilter filter_html but this leads to security concerns.

Conclusion

One can either use the textfiler filter_html or the teaser break functionality.