If you are getting this error suddenly and intermittently, this may be due to issues resolving localhost intermittently, perhaps intermittently when attempting IPv6 vs IPv4.
I was able to solve the problem by changing my references to localhost to 127.0.0.1 in my nginx configs.
Yuji Tomita 
We just moved to a cluster and got pages of these errors. This fixed it in a flash. Thank you so much.
I just had this issue less than a week ago, glad you were able to fix it!
I am using plesk and this has been a regular thing
What did you change to what?
localhost to 127….
or
127 to localhost?