I use yEDd version 3.18.2 on windows 10. As I'm not admin of my computer it is installed on my profile uner C:\Users\<login>\AppData\Roaming\yWorks\yEd
I just verified the files.. You were right! All my files are corrupted, I just downloaded one of them again and it works juste fine.
Many thanks for your quick answer, and sorry for this mistake which I should have checked by myself!