The Wii U 5.5.1 system update was released on January 11, 2016. This was released in Europe, North America & Japan.
Official USA change-log:
- Further improvements to overall system stability and other minor adjustments have been made to enhance the user experience
Only the VersionData title and the Internet Browser were updated.
The browser changes include an updated mvplayer.rpl, the changes in here include vuln fixes for libstagefright MP4 stsc and tx3g. However, it seems the browser still isn't using the latest libstagefright .git.
Other rpls were updated in the browser too.
System update report(s):