Steps to reproduce the issue

I am working on the latest staging. In the morning, when I return to the /administrator area, I get a fatal error message at the top of the login page.

If I try to login, it displays the warning "Your session has expired. Please log in again."

If I try to login again, I can finally access the backend site.

Expected result

No error messages. Login in a single step.

Actual result

 Fatal error: Uncaught exception 'RuntimeException' with message 'The session is not started.' in /..../libraries/joomla/session/handler/native.php on line 221 RuntimeException: The session is not started. in /..../libraries/joomla/session/handler/native.php on line 221 Call Stack: 7.9009 3325852 1. JSessionHandlerNative->save() /.../libraries/joomla/session/handler/native.php:0 

System information (as much as possible)

Latest J3.5 staging on PHP 5.6.16-2+deb.sury.org~trusty+1

Additional comments