CiviMail error due to unicode characters

The new AJAX-based saving (showing 'Saving' at right top corner) does not proceed at many of our cases of HTML-mail-content. Depending on the content and other (unknown) factors the saving just does not proceed. There is no Timout or worksround I can find. Thus saving mails and also saving by 'next step' or sending a test-email does not work.

I expect it is a problem at a client-side HTML-validation. BUT, the same HTML-content that works sometimes does not work at other times.

Attached a HTML-File that does not work (most of the times).

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