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.
I’d say that server restart after changes should be all you could potentially need to do, although I’m not overly familiar with plesk, perhaps there is more to that there.