Impossible to add a thumbnail to categories (for menu)

[PaulHenri]

Hi there,

I can't upload any thumbnail/images to categories.

I searched a lot online and it turns out every solutions has been include in new versions of Prestashop, so, whenever I check, the js is well fixed, the size issue is fixed, etc... 

But for me, it seems this issue is still happening, even though I am working on Prestashop 1.7.6.4 (and working on my VPS so everything is configurable).

Here is the error message I have :

"An unexpected error happened : Symfony\Component\Debug\Exception\ContextErrorException code 0"

I must point out that I already tried to turn on the PS_MODE_DEV to have more details on this error, but that didn't help (still the same error, no details added).

Thanks for your help,

Paul