What ever plugin you're using, check the permission nodes they provide, maybe there is a specific node you need to add to your default group.
Where can we actually check this? Does it differ depending on the server you are running?
It depends on the plugin you're running. Different plugins have different permissions.
@Cartnj, you're probably using a Chat Plugin which prevents players from talking. You'll either have to look at the mob's permissions and add those permissions to your Default group or just remove the plugin.