[Solved] White page after upgrading Piwik from 1.11 to 1.12

Hello,

I've recently upgraded from 1.11 to 1.12 and now Piwik shows a white page:
[attachment 1305 piwik_white_page.png]

I've followed the FAQ solutions and nothing worked, except checking web server error logs, I'm not sure where to find them (I'm under Ubuntu Server 10.04). I have also removed all piwik files and uploaded again (a lot of times) and it doesn't work. Don't know if it could be a hint but Piwik is taking its time loading the page, about 35 seconds (I'm using an old pc as a server, but it is taking 35 seconds!!)

Any hint why it is happening?

Thanks!