When I open the friends main feed page, no new posts of people I follow are shown.

Only posts that were added at the time I followed the people are shown.

My WordPress was recently installed, so it's up-to-date, and also the (dedicated) server is quite fast.

There are caches running, but the WP backend is included from caching and I tried to delete caches, which neither helped.

I also checked errors in Firefox developer tools, and in server error log, and found nothing.

Any idea what could be the problem? Maybe a folder within WordPress that needs special permissions?