Trying to use one-click-update I got a "500 internal server error". Ignoring that I accessed "index.php" without params.
PIWIK showed me that I have to update the DB, sounds like all could have worked. Clicking continue I got the message that I have to be logged in in order to update DBs. This is pretty weird, because I obviously was logged in and because I couldn't login(Piwik always redirected me to the DB-update site, even when i manually typed "module=Login" ).

So I first made a restore of my Piwik files from before the update. I tried to use one-click update a second time. Got a second 500 error. Interestingly I was able to perform DB update and could access Piwik afterwards.

But another problem arose: there were several error messages(ususllly 3), which told me "undefined index" in "Translate.php". I figured out that file integrety check failed for a whole bunch of files(including the .json-language files), which seemed to not have been updated.

So I decided to download RC1 from the Piwik servers and uploaded it via FTP to my installation. Finally all errors were gone and all works fine now.