avatar not displayed after cropping


I have been using the avatar plugin for my wordpress multisite as network activated plugin without any trouble till today.

But now, when I want to upload a new avatar image for one of my users:

- I get on 'modify user'
- then 'change avatar'
- I select the image file
- I clic on 'upload' button
- then I see the image displayed in the cropping window
- I clic on 'crop image' without making any crop
- then I get the message 'avatar updated'
- but I don't see any avatar image for the user

Could you help me finding what could cause the problem ?