So earlier, I tried to refer to DeathBlade/13.666 in this post. I originally tried to post the following:
The above screenshot includes the error message. Once the reference was removed, I was able to post.
As a control, here's a reference @Eris to demonstrate that I can refer to someone without special characters in their username.
I was able to reproduce the error for this post:
At DeathBlade/13.666's request, here's another reference /Aifa| (this doesn't work either)
Another one at DeathBlade/13.666's request: @Ωmega
This seems to work. It's probably specific to /.