My site is running with the last version of Wordpress and 15zine as a theme.
Sometimes when I surf through the pages Wordpress doesn't load the CSS correctly so I can see only the simple HTML page without the right formatting.
I've found out that these kinds of problems are usually caused by bad use of minimization and merging of cache plugins.
I would suggest to first disable both and start enabling only the minification. If it correctly works, I would go ahead with merging. With HTTP/2 merging is less needed