I upgraded from 2.6.5 to 2.7.1 and I now have the following Problem.
a.) My blog is on longer fixed width
b.) My blog is no longer centered (its now left)
c.) The header spreads over the entire browser window
The Problem is not related to the K2 theme I am using. If I switch to the classic theme or the default them the problem remains the same.
my blog is: http://www.baumgartners.at
All the best