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 an individual mod causing it, however there is no clear sign of which one. Googling the error showed up with a single thread where the mod in question was told to be reported to, which has led me here.