Something very similar happened to me on a website using qtranslate plugin for multilanguage content and admin panel.
By chance, I found that if I changed the admin language to english, the images uploaded fine, and the thumbs were created successfully.
But if the admin language is not set to english (in my case, spanish), when I try to upload images I get the same error symbols, and thumbs are not created.
I don’t have any further clue as to where the problem is and how to fix it. For the time being, I’m changing the admin panel to english everytime I need to upload images.
Hope this helps!