Caused by: java.net.ConnectException: Connection refused (Connection refused)
at java.net.PlainSocketImpl.socketConnect(Native Method) ~[?:1.8.0_144]
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350) ~[?:1.8.0_144]
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206) ~[?:1.8.0_144]
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188) ~[?:1.8.0_144]
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) ~[?:1.8.0_144]
at java.net.Socket.connect(Socket.java:589) ~[?:1.8.0_144]
at sun.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:673) ~[?:1.8.0_144]
at sun.net.NetworkClient.doConnect(NetworkClient.java:175) ~[?:1.8.0_144]
at sun.net.www.http.HttpClient.openServer(HttpClient.java:463) ~[?:1.8.0_144]
at sun.net.www.http.HttpClient.openServer(HttpClient.java:558) ~[?:1.8.0_144]
at sun.net.www.protocol.https.HttpsClient.(HttpsClient.java:264) ~[?:1.8.0_144]
at sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:367) ~[?:1.8.0_144]
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:191) ~[?:1.8.0_144]
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1181) ~[?:1.8.0_144]
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1032) ~[?:1.8.0_144]
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:177) ~[?:1.8.0_144]
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1546) ~[?:1.8.0_144]
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1474) ~[?:1.8.0_144]
at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:254) ~[?:1.8.0_144]
at com.mojang.authlib.HttpAuthenticationService.performGetRequest(HttpAuthenticationService.java:130) ~[patched_1.12.2.jar:git-Paper-1618]
at com.mojang.authlib.yggdrasil.YggdrasilAuthenticationService.makeRequest(YggdrasilAuthenticationService.java:66) ~[patched_1.12.2.jar:git-Paper-1618]
... 10 more

This issue is not related to ProtocolLib. Seems like your server is not able to communicate with mojang.