Sounds like the XHTML property type has lost its editor settings. http://i.imgur.com/es6OKI2.png

Go to admin mode > Edit custom property types > Xhtml string > Add setting.
Choose which TinyMCE buttons to use in the editor for XHTML properties.
Be sure that Dynamic Content is selected, then Save the settings.

(Beware these settings take effect for all XHTML properties globally on your site)

Thanks, that worked for me, but not one of the editors, who does not have admin access.