Thanks for your comment @arthur.grafo!
Is it really impossible to go back to my very first post (Sept 2017)?
There is a limit (100) for the Steem API function that I use and the input parameters (start_permlink, date) are not working correctly. When I've parsed and stored all post data in my own database I can load the posts from there and it will be possible to see all of them on SteemWorld.
When I've reached the last post SteemWorld can recover, would it not make more sense for the "Show More Results " to fade out, so that the user knows there is no point on clicking again?
This will also work as expected when I've changed the script to load the data from my server instead of the nodes. May still take a few weeks...