[SOLVED] backend add product image stopped working

-- HikaShop version -- : 2.6.1
-- Joomla version -- : 3.4.8

Location: backend
CSS: default
Custom HTML: none
Link: /administrator/index.php?option=com_hikashop&ctrl=upload&task=galleryimage&uploader=product&field=product_image&product_id=&product_type=product&file_type=product&tmpl=component

In the product add/edit screen (?option=com_hikashop&ctrl=product) the "+"-button in the section Images & Files to add an image from the server stopped functioning. I suspected this was after I installed attachinvoice.zip.
So I de-installed attachinvoice, removed all custom css and php from the server and installed a fresh Hikashop v2.6.1 (again) to be sure that everything is in the original state.

All this was to no avail. There has been no reason why this suddenly happens.
The popup to add a file works fine.

There must be something wrong with some javascript, but I wouldn't know where to look.
In Firefox I get no JS-errors. In Chrome I get 'webkitMovementX' is deprecated. Please use 'movementX' instead ; this message appears in Chrome JS-console in both add image and add file.

Where do I look for the solution to fix this?

Thank you Jerome. Found it: popup mode -> vex. I'll promise to RTFM in the future