Not possible to set avatar on forum

It is apparently not possible to set an avatar on this forum.
I always get:

Code:

The URL you entered for your avatar does not appear to be valid. Please ensure you enter a valid URL.

I get that message regardless of putting in a gravatar email address, a url to a 128x128 jpg or a url to a 128x128 png file.

You need to upload it from local storage, not set it through Avatar URL