Twitter summary card: issue with quotation marks


We have installed the required twitter tags for the summary card, but our website has not been accepted, although some articles have been validated correctly. It might be implicated by the Swiss quotation marks that are used in several articles:

(here: quotation marks in the title)

Could it help to provide the special characters using HTML-code or unicode in the appropriate twitter tags?

HTML-code: laquo raquo
Unicode: #171 #187

Thanks for your help!