Unable to upload any image to a product

-- HikaShop version -- : 2.6.0
-- Joomla version -- : 3.4.5
-- PHP version -- : 5.4.24
-- Browser(s) name and version -- : Firefox 42.0
-- Error-message(debug-mod must be tuned on) -- : Unable to upload any image to a product.

Hello,
After some time off I kept building my e-commerce website, currently using HikaShop Starter to lay down the fundamentals. When I logged in the Administrator side I was prompted to update both Joomla and HikaShop and so I did.
However while adding an image to a product, there is no way I can upload it. Drag and Drop does not work, not even the upload button. I click but literally nothing happens. I thought it was an issue with my browser, so I tried with different ones. Safari, Chrome and Firefox all have the same issue, and I never had such an issue months ago when I first tried HikaShop, so it's definitely not an issue with my computer.

Any way I can solve it? It's really frustrating and it's a bad enough issue to prevent me from using your extension properly. If I can't get rid of this issue I'll find myself forced to rebuild everything using something else.

Thanks in advance,
Riccardo