Piwik stops logging after i visited the dashboard

I have a strange problem with piwik:
Since i moved my server from Europe to US piwik stops logging for a site, if i login and open this site in the dashboard. For the rest of the day, the values (visitors, visits) does not change anymore. On the next day it works again until i login again.

timezone setting (php.ini) has to match the mysql server.

We plan to remove this limitation before 1.0.