Images not uploaded neither sent in 3.3.1

I’m experiencing a strange issue with images I’ve never seen.

  1. If I try to upload images in a message body selecting “Upload” -> select image the “Image properties” window gets stuck, not uploading anything and indicator spinning forever;

  2. If I link to an existing image (https://…) the back office body gets populated but when sending a test email the image is not there

Any clue?