Nginx + php-fpm - recv() error
Most probably you ran out of php-fpm workers. The log was silent because nothing was wrong in the code itself - workers just were busy processing your requests. If you did not get this on pages without MySQL queries, the bottleneck was the MySQL DB. You should identify long-running queries (using mytop or slow log feature or maybe some custom PHP logging around your SQL processing) and optimize them. Of course, "long queries" are actually quite short in high-load we world. Even 200ms query is long enough to put your server on its knees.