It was likely something in your last will or death note. When you close one of those notes and that warning message pops up, that means something in your note is triggering the filter.
These banned words tend to be homophobic or racist.
Without knowing what you had in your last will or death note, we cannot be sure what set the filter off. Could have been someone's name. Could have been a combination of a name plus other words.
N1: Ger
For example could set off the filter. The game thinks the player is trying to type a racist word.
incestigator
The "c" key is next to the "v" key. Sometimes people typo "invest" as "incest" and "incest" isn't a word that's allowed in the game.