Based on the information provided it's hard to judge. It could be anything between ELB, EC2 instance, OS and Application itself. I guess you reviewed Mendix console logs or even make correct setup and you send logs to CloudWatch.
Is the timeout between client and web server or between application and DB?
We had a look at ELB and our EC2 setup and logs and everything looks fine.
Yes, we have had a look at all the console logs.
The timeout is between client and web server.