I'm having the same issue, new domain with WP migrated with Duplicator plugin. At first I thought it was WAF, but it is disabled.
It's weird, sometimes files load just fine (css, js, images, etc), sometimes they don't. It's not consistent, and it's not all of them. Some of the times it even loaded the "default plesk" page instead of the wordpress one.
Using just nginx stops the problem (nginx settings > disable "Proxy mode"), and restarting apache stops the problem.
It looks to me it's related to some apache caching or something like that. Server load was low.