For those of you who are on shared hosting like OVH, please note that they sometimes enable the MAGIC_QUOTES option on PHP.
This can alter some admin functionnality, as the insertion of enhancers or video tags in the wysiwyg on a page.
Solution is simple, please add :
SetEnv MAGIC_QUOTES 0
at the begining of the .htaccess.shared-hosting file you must have renamed to .htaccess.