Resolution: Works As Intended
The website "skins.minecraft.net/MinecraftSkins/<PLAYERNAME>.png" is not working. If you try to to open for example "skins.minecraft.net/MinecraftSkins/iZePlayz.png" then it gives you a "404 Not Found" error.
This bug exist since one day. Two days ago the website has worked.
And yes i know i could get the skins over the new UUID Api. But the problem is, that many Programs or old Minecraft Version which dont have the UUID-System, are no longer working.