Varnish?

Hi @piska!

I am not that familiar with the settings of Varnish cache, but these kind of problems are usually related to Cache-Control response header. With certain settings this displays the cached HTML codes of pages without changing them. So even if a user logged in, it displays the not-logged in version of your pages. According to their documentation there is maybe a way to stop parts of their caching:
https://symfonycasts.com/screencast/symfonycon2018/varnish-caching-logged-in-users#avoiding-sessions
when Nextend Social Login starts its login flow, it sets a cookie:
SESSnsl

and later the WordPress auth cookie will also be set: https://developer.ww.wp.xz.cn/reference/functions/wp_set_auth_cookie/
and our cookie will be destroyed. I am not sure if this will be helpful information in resolving the problem or not, but I would rather suggest getting in touch with the host to find out how to fix this problem.