Collector does not load agents to the DT server. What could be the problem?
2018-04-19 15:28:07 INFO [AgentListener] Agent '172.25.48.42:37465' (version: 188.8.131.527) closed connection during handshake.
2018-04-19 15:28:09 INFO [CollectorAccess] Updating peer collector list: 
2018-04-19 15:28:17 WARNING [AgentPeer] Ignoring instrumentation rules for 'IB_SDP@testbed1:17555'.
Reason: No ClassLoader sensor found.
2018-04-19 15:28:17 INFO [com.dynatrace.diagnostics.a.f] Instrumentation caching enabled
2018-04-19 15:28:17 INFO [AgentPeer] IB_SDP@testbed1:17555 (Java, v184.108.40.2067, id=2d2e2365, profile=-, conf=-, group=-, starttime='Thu Apr 19 14:43:11 EAT 2018')
You have a below exception in the shared logs.
2018-04-19 15:25:22 WARNING [Collector] Could not resolve FQDN for IP '10.156.62.86', using fallback 'ugpdynatrace01' as hostname. Had following exception: com.dynatrace.diagnostics.sdk.net.FqdnResolutionException: Cannot resolve FQDN for IP address '10.156.62.86'
Didn't see any clear-cut issues that would cause this but it seems like it's normally a problem on the collector side. I'd do a collector restart and see if there's any issues with the agent connecting the next time it starts up. Additionally, there have been bugs in the past that could cause this that were resolved in updates so I'd make sure you're running up to date collector software especially.
Apart from that (unless others have ideas) you may want to open a support case.