I'm totally confused. I was away for a few hours and come back and now it works. I didn't change anything since the last time... very strange.
But to clear the things and answer your question. Maybe it helps another person:
I'm using Plesk on a Ubuntu server with apache and nginx as proxy. I played around with the WAF and the PHP settings, and one of this has to make the change, but maybe it needed a few hours to be in state.