Blank page after login from all browsers from every computer.

It's freshly installed Jenkins. Just after Setup Wizard with Install suggested plugins. Then in wizard dialogue user was created. And after that all attemts to login with this user lead to nothing.

For those who just need to fix it right now:

You can restart Jenkins with this URL: 

http://<jenkins.server>/restart

 or you can restart host, where Jenkins installed, or restart Jenkins in another way, depending of your installation.