Found the following in WildFly log:

13:43:43,914 ERROR [io.undertow] (default task-14) UT005085: Connection io.undertow.server.protocol.http2.Http2ServerConnection@3fcad868 for exchange HttpServerExchange{ GET /App/styles/app.css request {accept=[text/css,*/*;q=0.1], accept-language=[en-US,en;q=0.5], accept-encoding=[gzip, deflate, br], te=[trailers], user-agent=[Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:66.0) Gecko/20100101 Firefox/66.0], cookie=[__utma=111872281.1072418606.1556520732.1556520732.1556520732.2, __utmc=111872281, __utmz=111872281.1556520732.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none), __utmb=111872281.21.8.1556523801443, JSESSIONID=foShIHYwDFt3JUz9FI1yaretGFMc3gJE2Wdi5eog.app-node-00], referer=[https://localhost:8443/App/account.jsf], Host=[localhost:8443]} response {Expires=[], ETag=[app.css-20912-1550860946000], Last-Modified=[Fri, 22 Feb 2019 18:42:26 GMT], Cache-Control=[private], Pragma=[], Content-Type=[text/css], Date=[Mon, 29 Apr 2019 08:13:43 GMT], :status=[200]}} was not closed cleanly, forcibly closing connection

This is just downloading a static css file on loading web page. What may be wrong that connection was not closed cleanly?