Blank screen after session_start

I get a blank screen on a website. The website first works fine but today it gives a blank screen.

I get the following error from the session_start (i removed the "@")
Fatal error: Cannot instantiate abstract class User in /home/verbindingsmaatlat.nl/public_html/new/contao-new/system/initialize.php on line 75

When i deleted the cookies in the browser the website works fine for the first time i load the page. But when refreshing the page wthe gives a blank screen!

Hop you can help me out!

Which Contao version are you using? Did you validate the installation with the Contao check?