A little ashame I must admit that I found the error on my site. It was a linux webserver admin error, my own fault. The webserver did block all files that include the word “core” in it. Try it out yourself.
Enter the following adress https://yoursite/hellocore should not be blocked and the following adress https://yoursite/corehello should not either be blocked. I mistakenly blocked both files. I also blocked some the following wordpress files as well:
./wp-admin/includes/update-core.php
./wp-admin/update-core.php
./wp-includes/js/jquery/ui.core.js
The correct block was (in /etc/apache2/apache2.conf )
<Files ~ "^core$">
Order allow,deny
Deny from all
</Files>
The erroneous block that blocked wp-includes/js/jquery/ui.core.js was
<Files ~ "core">
Order allow,deny
Deny from all
</Files>
