Just a short note that may help some.
This morning, I could not access my piwik installation. Just a blank screen. No 404 or 403, no error message just a blank page.
I enabled the logs to “file”, but nothing.
Piwik has been updated to 1.6 a few days ago and everything went well so far.
I was able to fix the problem by resetting the /tmp folder.
I renamed the previous /tmp (for backup if something goes wrong), then created a new one, taking care to allow write access to the web server.