See https://github.com/MinecraftForge/MinecraftForge/pull/2886 for a detailed description.
Short version: Desktop.browse() does not like \.\ in URIs. Calling getCanonicalPath() on the screenshot file should fix this.
Comments 3
If this went from broken to working, wouldn't that imply that it was fixed, not "Cannot Reproduce" ?
It is definitely not working in 1.10.2.
Unable to reproduce in 16w32a in Windows 10.