Yeah that's a weird one. PSN's swear filter is a bit of a joke. I'm guessing it's too similar to the "n" word.
Here's a user-compiled list which includes "nicht" (deutsche for "no"?) - i don't know how that's profane but maybe night looks similar.
http://ps3trophies.com/forums/general-ps3-discussion/31203-list-words-can-t-used-psn.html
I found on the google a blacklist from GTA which doesn't include "night".
https://wccftech.com/list-banned-words-gta-online-sony-microsoft/
Swear words are funny.