It seems that some other process tried to target the TestHttpsServer with a plain connection:

----------System.out:(2/104)----------
Server started: listening on: 127.0.0.1:38468
client opening connection to: https://127.0.0.1:38468/d01
----------System.err:(11/965)----------
java.lang.RuntimeException: javax.net.ssl.SSLException: Unrecognized SSL message, plaintext connection?
at TestHttpsServer$ServerWorker.run(TestHttpsServer.java:458)
Caused by: javax.net.ssl.SSLException: Unrecognized SSL message, plaintext connection?
at java.base/sun.security.ssl.SSLEngineInputRecord.bytesInCompletePacket(SSLEngineInputRecord.java:146)
at java.base/sun.security.ssl.SSLEngineInputRecord.bytesInCompletePacket(SSLEngineInputRecord.java:64)
at java.base/sun.security.ssl.SSLEngineImpl.readRecord(SSLEngineImpl.java:605)
at java.base/sun.security.ssl.SSLEngineImpl.unwrap(SSLEngineImpl.java:499)
at java.base/sun.security.ssl.SSLEngineImpl.unwrap(SSLEngineImpl.java:475)
at java.base/javax.net.ssl.SSLEngine.unwrap(SSLEngine.java:634)
at TestHttpsServer$ServerWorker.run(TestHttpsServer.java:375)
STATUS:Failed.`main' threw exception: java.lang.RuntimeException: javax.net.ssl.SSLException: Unrecognized SSL message, plaintext connection?