Working here white 2.80 :)
Use Absolute URL's:
I made a folder in /Content called templates
in that i put a folder /lists
/content/templates:
your .html template's
/content/templates/lists:
template_list.js (or whatever you call it)
Containing:
var tinyMCETemplateList = [
// Name, URL, Description
["Newsletter Monthly", "http://www.your-domain.net/content/templates/news_monthly.html", "Monthly Newsletter."],
["Newsletter Special Offer", "http://www.your-domain.net/content/templates/news_special.html", "Special Offers."]
];
Now edit /Administration/Views/Shared/EditorTemplates/RichEditor.cshtml (line 41 if org.):
template_external_list_url: "http://www.your-domain.net/content/templates/lists/template_list.js",
Try it, it works for me