Did a lot of testing recently. It's the PHP 7.1.1 coming with the stack, which is full of memory leaking bugs. Even requests for static files increase the memory allocation uncontrollably if PHP 7.1.1 is installed, to the point PHP is unable to allocate more memory and the server crashes.
So, DON'T USE PHP 7.1.1 for production.
You can install PHP 5.6. PHP 7.1 also had numerous updates (7.1.8 currently), mostly dealing with memory leakages. You can try an up to date version.
Still strange, I'd expect some replies to people asking for help on server crashes in the recent years, as it's not a rare issue. At least a warning about 13.3 and PHP 7...