Last week I came across "HTTP 500 error" on my website and i managed to solve it by deleting all folders in theme directory and then reinstalling theme from admin panel.
Today I faced same issue. I don't think there is any problem with theme. Although by deleting theme folders i could solve problem.
My config: VPS Hosting: Ubuntu, Nginx, WP(latest); RAM: 1GB; CPU: 1; Storage: SSD; Theme: Ribon Lite; SSL: Cloudflare Free flexible SSL;
Today I increased memory limit to 500MB by editing wp-config.php. Is it possible problem maybe with memory limit. What can i do so i dont get into same problem again.