There is nothing about an incompatibility. The problem is dropbox recently made it harder to hotlink their files. In other words, the link that you tried to use was not really a hot-link.
But there is a simple workaround:
Take your dropbox link to the image. You will see there is a ?dl=0 part at the very end of the link. Replace that part with ?raw=1. (Do not run this new link on your browser's address bar because it will then redirect to another link which will not work). You can configure this new link as your profile picture or cover picture on permies.