Error saving layout overrides

-- HikaShop version -- : 2.6.1
-- Joomla version -- : 3.4.8
-- PHP version -- : 5.4.45
-- Browser(s) name and version -- : Opera 34.0
-- Error-message(debug-mod must be tuned on) -- : Method Not Implemented

GET to /administrator/index.php not supported.
Additionally, a 501 Method Not Implemented error was encountered while trying to use an ErrorDocument to handle the request.

Hello, I want to customize some views but when I try to use the editor in the back-end, in display>views, I get that error. I get it whether I hit save&close, save, close, only the help button doesn't do anything. Needless to say, the modifications aren't saved.

Also, I tried editing my overrides offline and uploading them via FTP to mytemplate>html>com_hikashop, but I can't find them to select them in the configuration. I cleaned cache on site and browser and they still don't show up.

I hadn't counted on this and I am against a deadline.
Please help!
Thanks!

Thank you for your reply, Nicolas!
I was missing a folder in the com_hikashop override. I should have put my "show_" files in a product folder to work. It's allright now, plus it seems it was a temporary server issue, as I'm not getting the error anymore either, without doing anything.

Irina