Could you set -Delastic.apm.log_level=debug and send the logs again? There's not much I can see from the logs other than that the agent successfully established a connection to APM Server:
2020-06-25 13:30:20,039 [main] INFO co.elastic.apm.agent.util.JmxUtils - Found JVM-specific OperatingSystemMXBean interface: com.sun.management.OperatingSystemMXBean
2020-06-25 13:30:21,884 [main] INFO co.elastic.apm.agent.configuration.StartupInfo - Starting Elastic APM 1.17.0 as SIFU on Java 1.8.0_242 (Oracle Corporation) Linux 3.10.0-514.16.1.el7.x86_64
2020-06-25 13:30:21,885 [main] WARN co.elastic.apm.agent.configuration.StartupInfo - Detected usage of an old configuration key: 'server_url'. Please use 'server_urls' instead.
2020-06-25 13:30:21,922 [main] INFO co.elastic.apm.agent.impl.ElasticApmTracer - Tracer switched to RUNNING state
2020-06-25 13:30:22,808 [elastic-apm-server-healthcheck] INFO co.elastic.apm.agent.report.ApmServerHealthChecker - Elastic APM server is available: {"ok":{"build_date":"2019-06-18T12:11:59Z","build_sha":"2e58a05c97d4551aca59ef09aff651c79ea3f4db","version":"6.8.1"}}
Which web framework are you using in your spring boot application? Is it Spring MVC on a Tomcat server?
Could you test if version 1.15.0 is working for you as expected? Is there a chance you could test with a more recent version of APM Server?