I've been getting this error when stopping my Forge server, which seems to be the cause of it hanging for varying periods (sometimes 1 second, sometimes forever).
Here is the gist of it (errors are at the very bottom of the file): https://gist.github.com/MarioSMB/26c9f7bef085a113ede1b0240bb85f3d
I created a bug report on the tracker, which suggested I open a forum thread for the issue instead: https://github.com/MinecraftForge/MinecraftForge/issues/7704
Given some other mods are showing similar errors, I also reported it to their trackers. Some have suggested this to be a