So it looks like MacOS Sierra broke pbcopy/pbpaste for tmux. This manifests as clipboard errors when using netrw/vinegar in neovim under tmux. (Dunno why netrw is writing to the clipboard but anyways).
The solution is to use Homebrew to reinstall a patched version of reattach-to-user-namespace:
brew uninstall reattach-to-user-namespace brew install reattach-to-user-namespace --with-wrap-pbcopy-and-pbpaste
Although this was annoying, it did lead to running brew cleanup which reclaimed 3.8GB of disk space. Jinkies!
When using the plugin plantuml-syntax, add the following one-liner in .vimrc to render a PNG in the current directory when ‹leader›e is pressed:
autocmd FileType plantuml nnoremap e :!java -jar ~/bin/plantuml.jar -o %:p:h %