No logging of visits

For no apparent reason (no server changes or other) Piwik 0.2.37 does not log visits any more (past two days) for the two sites I monitor.
What could be the problem?
Thank you very much for your help!

Please check your error logs.

Also, try accessing piwik.php?idsite=1 from the browser to make sure you’re not running up against something in .htaccess, mod_rewrite, mod_security, …(ad infinitum)…

I had exactly the same problems since Tuesday evening. The reason for that was a PHP Update my Provider made (dF - is this yours, too? style_emoticons/<#EMO_DIR#>/rolleyes.gif ). I’ve just installed Piwik again and kept the existing database - now everything’s fine.

Thank you all for your help and ideas!
The GeoIP widget would not work as expected and caused PIWIK not to log anything. Saw a broken PIWIK images accidently when checking the page with Google Chrome.