I overlooked something peculiar, too: the messages isn't in the colour other "Unknown command" messages appear. It uses two colours, none of which is assigned to such an event.
[color:yellow].----'-/'[/color][color:green]-/ Unknown command[/color]
The first characthers (.----'-/') are yellow, the rest (-/ Unknown command) is green.
I also checked back in the logs, I succesfully used a /say singleline alias before I replace the non breaking spaces by normal ones, and the same alias produces the same error now.