AMI Linux 2 EC2 issues - fork: retry: No child processes | Resource temporarily unavailable

It sounds like there is something on the instance which is starting a lot of processes. I would recommend that you raise a support case because there may be other troubleshooting steps that the support team can help with.

To troubleshoot further though, given that it sounds like there is something awry on the instance, I would add additional logging to see what is happening. If it continues to occur, build a new instance from the AMI; make sure that it works correctly; then compare the two.