Looking a bit closer at the logs (lots of it with DEBUG enabled :), I noticed the following earlier in the HTTP/2 conversation:
[qtp1612799726-17] DEBUG org.eclipse.jetty.http2.hpack.HpackEncoder - encode IdxFieldS1:':status: 200' to '88'
[qtp1612799726-17] DEBUG org.eclipse.jetty.http2.HTTP2Flusher - Failure generating HeadersFrame@11e995ab#1{end=false}
org.eclipse.jetty.http2.hpack.HpackException$SessionException: Could not hpack encode HTTP/2.0{s=200,h=2,cl=268}
at org.eclipse.jetty.http2.hpack.HpackEncoder.encode(HpackEncoder.java:283)
at org.eclipse.jetty.http2.generator.FrameGenerator.encode(FrameGenerator.java:61)
at org.eclipse.jetty.http2.generator.HeadersGenerator.generateHeaders(HeadersGenerator.java:70)
at org.eclipse.jetty.http2.generator.HeadersGenerator.generate(HeadersGenerator.java:57)
at org.eclipse.jetty.http2.generator.Generator.control(Generator.java:91)
at org.eclipse.jetty.http2.HTTP2Session$ControlEntry.generate(HTTP2Session.java:1216)
at org.eclipse.jetty.http2.HTTP2Flusher.process(HTTP2Flusher.java:211)
at org.eclipse.jetty.util.IteratingCallback.processing(IteratingCallback.java:237)
at org.eclipse.jetty.util.IteratingCallback.iterate(IteratingCallback.java:219)
at org.eclipse.jetty.http2.HTTP2Session.frame(HTTP2Session.java:778)
at org.eclipse.jetty.http2.HTTP2Session.frames(HTTP2Session.java:750)
at org.eclipse.jetty.http2.HTTP2Stream.send(HTTP2Stream.java:135)
at org.eclipse.jetty.http2.server.HttpTransportOverHTTP2.lambda$sendHeaders$0(HttpTransportOverHTTP2.java:212)
at org.eclipse.jetty.http2.server.HttpTransportOverHTTP2$TransportCallback.send(HttpTransportOverHTTP2.java:436)
at org.eclipse.jetty.http2.server.HttpTransportOverHTTP2.sendHeaders(HttpTransportOv
