That’s funny you bump this topic, because I was reading it again this morning.
I actually (again) encountered the same issue yesterday.
I noticed that download_remote_images_to_local was set to false for 3 months on my instance, but however, this time again, I don’t recall having seen a warning. 
I’m not sure why. I looked in my inbox, but couldn’t find any related message, including in the archives.
I didn’t see a similar message on my instance but let’s ignore this.
What is unclear in the message you had? It seems pretty straightforward to me. As well as for the solution: free some disk space.