AppEngine - Client closed keepalive connection
Solution 1:
As clarified in this other similar case here, this type of issue, indeed, is related to the code, where it's not well written and then, the runtime is caughting the exception from it. In this situation, it's recommended to take a look at your code and look for exceptions that are not being handled by your code.
An alternative is to check in the Cloud Dashboard - as mentioned here - for more details in the Error Reporting, so you can get more information from the stack trace.