If I'm reading that right, you're trying to set an avatar via URL, right?
There seems to be a restriction/issue with SMF (forum software), that only allows HTTP links. You can try and provide the Dropbox link as HTTP instead of HTTPS, which does a redirect and which should work (that's at least what I'm doing, but with my own server and not Dropbox).
Alternatively you can upload the image to the forum itself.