"Page URL not defined" shown in Visitor Log after 2.0.3 updated

After having updated from 1.12 to 2.0.3, there are many records “Page URL not defined” shown in the Visitor Log. It often happens to Firefox only, but not always. I’ve used Chrome/IE, but no this problem.

In the Piwik upgrade process, it was very smooth. I’d rather downgrade the Piwik but cannot enjoy new features.

what server environment and php version are you running?