Increase logging/tracing on Oracle 12c database
You need to enable listener logging.

In your listener.ora, set:

LOG_DIRECTORY_listener=/path/to/your/oracle/home/network/admin/log
LOG_FILE_listener=listener.log
TRACE_DIRECTORY_listener=/path/to/your/oracle/home/network/admin/trace
TRACE_FILE_listener=listener.trc
TRACE_LEVEL_listener=10


... make sure you substitute your listener name for listener above.

You can also use ADR parameters instead - see the documentation.