Okay, I did it in the same way (without changing the language). I upload a new image and close the popup window. I saw my new avatar on my profile. I refresh my profile via [ctrl] + [F5] (windows system) and my new avatar is still shown. I click on 'edit avatar' and the popup shows my new avatar, too.

And you're using the latest version of Opera or Chrome?

No, it works as expected. Again, you have to clean-up your browser cache.

I've cleaned the cache, I've even reinstalled the browser. It's not just my problem but all my users since 2.3.7

Can you explain why mylittleforum.net does not have the problem then? yet your 2.3.7 and my 2.3.7 site has the problem.

If it was a cache issue this site would do the same as ours.