A Microsoft file hosting and synchronization service.
I FOUND A REAL WORK-AROUND!!!
The shared folder name was having a space in it and I just renamed it to remove the space! I don't know if it was because there was a space or if renaming it is enough to fix the problem, but after having a VERY ANNOYING .url file since 2 months, it's now back synchronized on my PC as before!
Great.