After automatic upgrade I saw only a blank page (with only the title on top). No error message. I tried to overwrite the files manually from the download, no change.
Finally I got Piwik running again by overwriting config.ini with config.ini.old. The upgrading continued then with the database upgrade, and now I see the UI again.
Not sure if there is not something missing in the new config.ini. The live visitor log seems to be broken now:
WARN Live[2014-08-20 21:28:15] [afb39] Failed to get data from API: Referrer type '' is not valid.#0 /.../plugins/Live/Visitor.php(359): Piwik\Plugins\Referrers\getReferrerTypeFromShortName(NULL)#1 /.../plugins/Live/Visitor.php(109): ...