"There has been an error cropping your image"

Hi there,

Strange issue right there.
I can’t crop images using “Select and Crop” function while editing a new theme. I can’t figure why, after lots of searching.
I can edit pictures from Media (even crop) with no problem, so i guess my file permissions are right.

The context might be important.

– Dedicated server, Debian 8, 128GB RAM, Xeon Hexacore, 1000mbits/s.
– Hosting several WordPress websites.
– I recently Migrated everything from a server running Debian 7 to this one.
– The two websites I tested have the latest version of WP and have the exact same issue (but had about the same migration).
– I’m not sure if it worked before migration as i didn’t setup websites for a while but i guess it did work

What i’ve done :
Of course apt-get update && apt-get upgrade
Check that php5-db is already installed (it was), then I checked PHP Info and to my semi-pro eye, everything seemed fine.
I tried cropping from several theme previews, same result.
I tried disabling plugins that could interfere : Query Monitor, Regenerate Thumbnails, Wordfence, WP User Avatar…
I even tried with no plugin at all on the smallest website (and still no luck).
I checked the var/log/apache2/error.log : Nothing about that error.
I tried restarting apache2, just in case… Nothing.

I’m completely stuck, it may be a wordpress issue or just me missing something…
Any help would be very appreciated.
Thanks for your attention.