This sounds like the same issue I reported in January, where I reported that this was happening when you use the “Choose or upload an image” button.
Stefano’s response was “It’s a library of wordpress that, on some installation, does not work. And I was unable to fix it..BUT don’t worry, I’m releasing a fix on my plugin with a different code to manage the images. In the meanwhile you can insert an image using the image button on the editor toolbar a copying there the image url uploaded.”
Stefano, has this fix been released yet?