doesn't work, send back to index.php but nothing is happening

Don’t get any errors?

No errrors, just a blank page.

Running on PHP5 with curl support enabled?

yes

Did you try adding
define('WP_DEBUG', true);
to your wp-config file. Maybe that reveals some error messages.

put a curl-script on the server, check:
https://marcoraaphorst.nl/checkcurl.php

I get errors on several plugins. But this breaks the site for viewers. How can I hide these errors for normal visitors?

if(is_admin())
   define('WP_DEBUG', true);

should do the trick.

that code gives error:
Fatal error: Call to undefined function is_admin() in /home/offline/domains/marcoraaphorst.nl/public_html/wp-config.php on line 17

Add the code to your theme’s functions.php file. ??

I did that. Still blank page

Hmm, can you provide me a backend admin login so I can have a look myself? Login data please to scripts (at) schloebe (dot) de