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.
Also reproduced by other users.
In d730096437a840ec25ff4b10641dc990a7064a8d: Refs #4702 Setup access state