I think it's same issue has UNDERTOW-592, but i cannot re-open it and nobody responds to my comment.
450+ crash in one day...

My Undertown modules = undertow-core-1.3.15.Final, undertow-servlet-1.3.15.Final, undertow-websockets-jsr-1.3.15.Final

Exemples:

2016-10-22 09:23:00,764 ERROR [io.undertow.request] UT005001: An exception occurred processing the request: java.net.URISyntaxException: Relative path in absolute URI: https://X.X.X.X:443http://X.X.X.X:80/phppma/

at java.net.URI.checkPath(URI.java:1823)
at java.net.URI.<init>(URI.java:672)
at io.undertow.security.handlers.SinglePortConfidentialityHandler.getRedirectURI(SinglePortConfidentialityHandler.java:51)
at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.getRedirectURI(ServletConfidentialityConstraintHandler.java:83)
at io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:49)
at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.handleRequest(ServletConfidentialityConstraintHandler.java:64)
at io.undertow.servlet.handlers.security.ServletSecurityConstraintHandler.handleRequest(ServletSecurityConstraintHandler.java:59)
at io.undertow.security.handlers.AuthenticationMechanismsHandler.handleRequest(AuthenticationMechanismsHandler.java:60)
at io.undertow.servlet.handlers.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:77)
at io.undertow.security.handlers.NotificationReceiverHandler.handleRequest(NotificationReceiverHandler.java:50)
at io.undertow.security.handlers.AbstractSecurityContextAssociationHandler.handleRequest(AbstractSecurityContextAssociationHandler.java:43)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at org.wildfly.extension.undertow.security.jacc.JACCContextIdHandler.handleRequest(JACCContextIdHandler.java:61)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:292)
at io.undertow.servlet.handlers.ServletInitialHandler.access$100(ServletInitialHandler.java:81)
at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:138)
at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:135)
at io.undertow.servlet.core.ServletRequestContextThreadSetupAction$1.call(ServletRequestContextThreadSetupAction.java:48)
at io.undertow.servlet.core.ContextClassLoaderSetupAction$1.call(ContextClassLoaderSetupAction.java:43)
at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:272)
at io.undertow.servlet.handlers.ServletInitialHandler.access$000(ServletInitialHandler.java:81)
at io.undertow.servlet.handlers.ServletInitialHandler$1.handleRequest(ServletInitialHandler.java:104)
at io.undertow.server.Connectors.executeRootHandler(Connectors.java:202)
at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:805)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

2016-10-22 09:23:00,593 ERROR [io.undertow.request] UT005001: An exception occurred processing the request: java.net.URISyntaxException: Relative path in absolute URI: https://X.X.X.X:443http://X.X.X.X:80/phpmy/
...

2016-10-22 09:23:00,423 ERROR [io.undertow.request] UT005001: An exception occurred processing the request: java.net.URISyntaxException: Relative path in absolute URI: https://X.X.X.X:443http://X.X.X.X:80/2phpmyadmin/
...

2016-10-22 09:23:00,423 ERROR [io.undertow.request] UT005001: An exception occurred processing the request: java.net.URISyntaxException: Relative path in absolute URI: https://X.X.X.X:443http://X.X.X.X:80/2phpmyadmin/
...

2016-10-22 09:23:00,253 ERROR [io.undertow.request] UT005001: An exception occurred processing the request: java.net.URISyntaxException: Relative path in absolute URI: https://X.X.X.X:443http://X.X.X.X:80/phpmyadmin4/
...

2016-10-22 09:23:00,083 ERROR [io.undertow.request] UT005001: An exception occurred processing the request: java.net.URISyntaxException: Relative path in absolute URI: https://X.X.X.X:443http://X.X.X.X:80/phpmyadmin3/
...

2016-10-22 09:22:59,573 ERROR [io.undertow.request] UT005001: An exception occurred processing the request: java.net.URISyntaxException: Relative path in absolute URI: https://X.X.X.X:443http://X.X.X.X:80/phpMyAdmin/
...

2016-10-22 09:22:59,062 ERROR [io.undertow.request] UT005001: An exception occurred processing the request: java.net.URISyntaxException: Relative path in absolute URI: https://X.X.X.X:443http://X.X.X.X:80/mysql/mysqlmanager/
...