For anyone who got a problem with the Shoutbox, press CTRL + F5.
Thanks to SuperNovaAO and have fun trolling again
happens with all MyBB forums after update 1.6.3
It was never broken, just some clientside caching problem.
Ctrl+F5 will fix it, also changing a line in your templates would help it (static include -> static_file.js?version=1603)