Found this post, will maybe solve the problem, will have the answer tomorrow morning...
https://talk.plesk.com/threads/502-bad-gateway-everyday.336609/
Bonjour,
Every morning from 3 days at 6.30 (french time) all the websites installed on the webserver show error 502 for 20 minutes to 1 hour. The error is fixed when I restart apache or fixed by itsefl after 20min to 1 hour.
Is someboday have the same problem or know how to fix this problem...