Active PHP session detected

I started to change my theme from TwentyTen to Raft (to get into full site editing)

But after the first step, the site health status was:

“A PHP session was created by a session_start() function call. This interferes with
REST API and loopback requests. The session should be closed by
session_write_close() before making any HTTP requests.”

How do I close where??

Thank you in advance for any hint…

The page I need help with: [log in to see the link]

The page I need help with: [log in to see the link]