I'm pretty sure they're banning the letter N because it was the 14th "proclamation" in the recent posting about constitutional changes by Xinhua news. They tried to bury it under a set of 20~ changes and disguise the wording. First, 14 was banned on weibo. Then, N was banned as people quickly jumped to the 14th letter of the English alphabet.
Weibo also has a history of doing stuff like this, Tiananmen Square is sometimes referred to as May 35th, so they like to play with numbers and their meanings.