I tried to save same changes in global jenkins config and have this error with Reverse Proxy Auth plugin installed, can u check ? :

Stack trace

java.lang.UnsupportedOperationException: Refusing to marshal org.acegisecurity.GrantedAuthorityImpl for security reasons; see https://jenkins.io/redirect/class-filter/ at hudson.util.XStream2$BlacklistedTypesConverter.marshal(XStream2.java:543) at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69) at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58) at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:43) at com.thoughtworks.xstream.core.AbstractReferenceMarshaller$1.convertAnother(AbstractReferenceMarshaller.java:88) at com.thoughtworks.xstream.converters.collections.AbstractCollectionConverter.writeItem(AbstractCollectionConverter.java:64) at com.thoughtworks.xstream.converters.collections.ArrayConverter.marshal(ArrayConverter.java:45) at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69) at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58) at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:43) at com.thoughtworks.xstream.core.AbstractReferenceMarshaller$1.convertAnother(AbstractReferenceMarshaller.java:88) at com.thoughtworks.xstream.converters.collections.AbstractCollectionConverter.writeItem(AbstractCollectionConverter.java:64) at com.thoughtworks.xstream.converters.collections.MapConverter.marshal(MapConverter.java:79) at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69) at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58) at com.thoughtworks.xstream.core.AbstractReferenceMarshaller$1.convertAnother(AbstractReferenceMarshaller.java:84) at hudson.util.RobustReflectionConverter.marshallField(RobustReflectionConverter.java:265) at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:252) Caused: java.lang.RuntimeException: Failed to serialize org.jenkinsci.plugins.reverse_proxy_auth.ReverseProxySecurityRealm#authContext for class org.jenkinsci.plugins.reverse_proxy_auth.ReverseProxySecurityRealm at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:256) at hudson.util.RobustReflectionConverter$2.visit(RobustReflectionConverter.java:224) at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.visitSerializableFields(PureJavaReflectionProvider.java:138) at hudson.util.RobustReflectionConverter.doMarshal(RobustReflectionConverter.java:209) at hudson.util.RobustReflectionConverter.marshal(RobustReflectionConverter.java:150) at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69) at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58) at com.thoughtworks.xstream.core.AbstractReferenceMarshaller$1.convertAnother(AbstractReferenceMarshaller.java:84) at hudson.util.RobustReflectionConverter.marshallField(RobustReflectionConverter.java:265) at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:252) Caused: java.lang.RuntimeException: Failed to serialize jenkins.model.Jenkins#securityRealm for class hudson.model.Hudson at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:256) at hudson.util.RobustReflectionConverter$2.visit(RobustReflectionConverter.java:224) at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.visitSerializableFields(PureJavaReflectionProvider.java:138) at hudson.util.RobustReflectionConverter.doMarshal(RobustReflectionConverter.java:209) at hudson.util.RobustReflectionConverter.marshal(RobustReflectionConverter.java:150) at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69) at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58) at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:43) at com.thoughtworks.xstream.core.TreeMarshaller.start(TreeMarshaller.java:82) at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.marshal(AbstractTreeMarshallingStrategy.java:37) at com.thoughtworks.xstream.XStream.marshal(XStream.java:1026) at com.thoughtworks.xstream.XStream.marshal(XStream.java:1015) at com.thoughtworks.xstream.XStream.toXML(XStream.java:988) at hudson.XmlFile.write(XmlFile.java:193) Caused: java.io.IOException at hudson.XmlFile.write(XmlFile.java:200) at jenkins.model.Jenkins.save(Jenkins.java:3168) at jenkins.tools.GlobalToolConfiguration.configure(GlobalToolConfiguration.java:92) at jenkins.tools.GlobalToolConfiguration.doConfigure(GlobalToolConfiguration.java:79) at java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:627) at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:343) at org.kohsuke.stapler.interceptor.RequirePOST$Processor.invoke(RequirePOST.java:77) at org.kohsuke.stapler.PreInvokeInterceptedFunction.invoke(PreInvokeInterceptedFunction.java:26) at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:184) at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:117) at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:129) at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58) at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:715) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:845) at org.kohsuke.stapler.MetaClass$10.dispatch(MetaClass.java:374) at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:715) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:845) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649) at org.kohsuke.stapler.Stapler.service(Stapler.java:238) at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:841) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1650) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:154) at hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:59) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151) at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:157) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637) at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:99) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637) at org.jenkinsci.plugins.reverse_proxy_auth.ReverseProxySecurityRealm$1.doFilter(ReverseProxySecurityRealm.java:514) at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637) at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637) at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637) at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637) at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:533) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143) at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:524) at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132) at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:190) at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1595) at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:188) at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1253) at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:168) at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:473) at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1564) at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:166) at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1155) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141) at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132) at org.eclipse.jetty.server.Server.handle(Server.java:564) at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:317) at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:251) at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:279) at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:110) at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:124) at org.eclipse.jetty.util.thread.Invocable.invokePreferred(Invocable.java:128) at org.eclipse.jetty.util.thread.Invocable$InvocableExecutor.invoke(Invocable.java:222) at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:294) at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:199) at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at java.lang.Thread.run(Thread.java:745)