502 bad gateway Error is a generic error that indicates that the problem lies in the communication with the webserver. The error message does not give any hint about the root cause of the problem. When this error occurs on a website an error message like this appears.
There are many reasons that cause this error. There is a chance that the server has some extra load. Another possible cause is the presence of some network issues between the two servers. If this is the case then this error is a temporary one. At times an improperly configured firewall can cause this error. And if there is a coding error then also the chances are high for this error occurrence.