how do fix Cannot start session when headers already sent

I had a similar problem with this topic: https://ww.wp.xz.cn/support/topic/how-to-troubleshoot-when-session-identifier-is-missing/

I have also run the WSL Diagnostic Tool and gotten all green OK: s on each row.

Still, when I try to use WSL a new window pops up saying contacting Facebook or Google and then after a few seconds a black error page is shown (https://screenpresso.com/=AoeWc) saying:
“WordPress Social Login Endpoint.
406. The session identifier is missing.
For more information refer to WSL Troubleshooting. ”

debug log: PHP Warning: session_start (): Cannot start session when headers already sent in /home/244582.cloudwaysapps.com/tefajgsbvm/public_html/wp-content/plugins/wordpress-social-login/wp-social-login.php on line 64

I do have Facebook-networks set up, and the problem arises.

This topic has been resolved (https://ww.wp.xz.cn/support/topic/how-to-troubleshoot-when-session-identifier-is-missing/), but there is no description how to do it? Can anyone help me? Please.