Oh, what a mess! I have never before had such trouble with upgrading to a new version. White page when trying to call Piwik all the time. Changed CHMOD to 755/777 for all tmp directories without success. Completely de-installed and re-installed. No success. Created a new piwik directory and re-installed without success.
A nightmare! Wished I could have 2.5 back 
Found an old 2.4.1 version and re-installed it sucessfully. Should I dare try another upgrade to 2.6.1…? 
Tried an automated upgrade to 2.6.1 afterwards…white page again…what a shit (td)
Which Version of PHP are you using?
It´s PHP version 5.3.2 according to phpinfo.php
Please see Upgrade from 2.5.0 to 2.6.1. breaks · Issue #6134 · matomo-org/matomo · GitHub
That issue is already fixed. You can try to apply the changes manually or wait for the bugfix release
Honestly speaking, I do not know how/where I could fix this manually as I do not find a Factory.php in the directory ‘core’.
Think it might be better to wait for 2.6.2, right?
@Paul right, or simply use 2.5.0 which will work fine in the meantime http://builds.piwik.org/piwik-2.5.0.zip
You can now also use the new beta 2.7.0-b1. Which can be downloaded on our build server aswell.