After the update from Jenkins 2.27 to 2.28, webhooks are not working any longer. Gitlab calls the URL "https://<Jenkins URL>/gitlab/build_now". This call fails with the following error.

java.lang.AbstractMethodError
at org.kohsuke.stapler.lang.Klass.getDeclaredFields(Klass.java:59)
at org.kohsuke.stapler.lang.Klass.getFields(Klass.java:70)
at org.kohsuke.stapler.KlassDescriptor.<init>(KlassDescriptor.java:26)
at org.kohsuke.stapler.MetaClass.buildDispatchers(MetaClass.java:105)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:93)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:91)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:204)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:218)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:711)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:686)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.MetaClass$10.dispatch(MetaClass.java:362)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:746)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649)
at org.kohsuke.stapler.Stapler.service(Stapler.java:238)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:812)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1669)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:135)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:126)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:86)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:117)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:93)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:67)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:553)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
at org.eclipse.jetty.server.Server.handle(Server.java:499)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:311)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)
at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)