I applied the timeout settings listed in this forum post: https://forum.jmix.io/t/jmix-1-x-app-not-working-on-aws-communication-error-invalid-json-from-server-1-x/4460/24
My settings are:
proxy_connect_timeout 600s;
proxy_read_timeout 900s;
proxy_send_timeout 900s;
This worked on my AWS Beanstalk deployment which includes nginx.