Pages not updating after one day

I have installed Piwik over the weekend and at first it seemed to be working great.
After on day however the data on the web pages is not being updated anymore, although I can see pages being added to the table “piwik_log_action”.
Can anyone tell me how to trace this behavior? Where can I look to see why the stats are not updated anymore?

I am running Piwiki 0.4.2 under the following configuration:
Ubuntu 9.04 64bit
Intel Q9300
MySQL 5.1
GD 2.0.36
PHP 5.2.6

Any help is very much appreciated.