First, don't run what you run ... did some googling, however ... that could be dangerous ... and found a StackOverFlow discussion about Ubuntu:
While the above isn't specific to Moodle, there was one responder in the above discussion that suggested restarting fastcgi. Since you are running nginx, didn't mention what running for PHP under nginx.
service php-fastcgi restart
The discussion also said that wasn't really a fix as the error might occur again later, but ....
'spirit of sharing', Ken