OK, I’ve checked the page again and now all links of the particular news post have the attributes added. The internal link gets converted to:
<a href="7E1" mce_href="/">Leitbild</a>
The site in question is hxxp://www.artefactur.com, look for news post "Artefactur ist jetzt eine UG". (Please don’t use IE to view the page yet

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- Just because you're not paranoid doesn't mean they're not after you -
To disable the "mce_" tags when cleanup is turned off, replace "assets/plugins/tinymce3241/jscript/tiny_mce/tiny_mce.js" with this code. I have not tested this in all situation please report any quirks. The solution disables "processHTML:function" except b,i are still replaced with strong,em