Wrong data in dashboard



we use Piwik Version 2.2.2. On the dashboard and on the “Visitor Log” appears additional to the vistited pages with real time and date several entries with date “Fri 20 May”.

The entries in “Action -> Pages” are correct. There are only the correct visits displayed. I’ve searched in the database: I haven’t found any entries matching the date.

What could cause that problem?