The portals (both of them) work, but I cannot get to the Hosts>Host setting page.  I recently upgraded in place to 4.9.1 and think I am using DNNFriendlyUrl default. I have not made mods.   Did I need to change something in web.config for DNNFriendlyUrl during this update? I only made edits for the connection string and machine code sections.  TIA!

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Error: Host Settings is currently unavailable.
DotNetNuke.Services.Exceptions.ModuleLoadException: D:\dnnweb\ipcm\admin\Host\FriendlyUrls.ascx.vb(44): error BC30560: 'RewriterRuleCollection' is ambiguous in the namespace 'DotNetNuke.HttpModules.Config'. ---> System.Web.HttpParseException: D:\dnnweb\ipcm\admin\Host\FriendlyUrls.ascx.vb(44): error BC30560: 'RewriterRuleCollection' is ambiguous in the namespace 'DotNetNuke.HttpModules.Config'. ---> System.Web.HttpCompileException: D:\dnnweb\ipcm\admin\Host\FriendlyUrls.ascx.vb(44): error BC30560: 'RewriterRuleCollection' is ambiguous in the namespace 'DotNetNuke.HttpModules.Config'. at System.Web.Compilation.BuildManager.PostProcessFoundBuildResult(BuildResult result, Boolean keyFromVPP, VirtualPath virtualPath) at System.Web.Compilation.BuildManager.GetBuildResultFromCacheInternal(String cacheKey, Boolean keyFromVPP, VirtualPath virtualPath, Int64 hashCode) at System.Web.Compilation.BuildManager.GetVPathBuildResultFromCacheInternal(VirtualPath virtualPath) at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) at System.Web.UI.BaseTemplateParser.GetReferencedType(VirtualPath virtualPath, Boolean allowNoCompile) at System.Web.UI.BaseTemplateParser.GetUserControlType(VirtualPath virtualPath) at System.Web.UI.MainTagNameToTypeMapper.ProcessUserControlRegistration(UserControlRegisterEntry ucRegisterEntry) at System.Web.UI.BaseTemplateParser.ProcessDirective(String directiveName, IDictionary directive) at System.Web.UI.TemplateControlParser.ProcessDirective(String directiveName, IDictionary directive) at System.Web.UI.TemplateParser.ParseStringInternal(String text, Encoding fileEncoding) --- End of inner exception stack trace --- at System.Web.UI.TemplateParser.ProcessException(Exception ex) at System.Web.UI.TemplateParser.ParseStringInternal(String text, Encoding fileEncoding) at System.Web.UI.TemplateParser.ParseString(String text, VirtualPath virtualPath, Encoding fileEncoding) at System.Web.UI.TemplateParser.ParseFile(String physicalPath, VirtualPath virtualPath) at System.Web.UI.TemplateParser.ParseInternal() at System.Web.UI.TemplateParser.Parse() at System.Web.Compilation.BaseTemplateBuildProvider.get_CodeCompilerType() at System.Web.Compilation.BuildProvider.GetCompilerTypeFromBuildProvider(BuildProvider buildProvider) at System.Web.Compilation.BuildProvidersCompiler.ProcessBuildProviders() at System.Web.Compilation.BuildProvidersCompiler.PerformBuild() at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath) at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) at System.Web.UI.TemplateControl.LoadControl(VirtualPath virtualPath) at System.Web.UI.TemplateControl.LoadControl(String virtualPath) at DotNetNuke.UI.Skins.Skin.InjectModule(Control objPane, ModuleInfo objModule, PortalSettings PortalSettings) --- End of inner exception stack trace ---

Solution: I removed the file "'DotNetNuke.HttpModules.UrlRewrite.dll' from the /bin folder and the Host>Host Setting page works again.

Thank you Alex!  I was not aware I was using anything other than files from core DNN upgrades. I look now and see I have many DLLs in /bin which are not in the /bin of the current upgrade 4.9.1 download. My portal is two years old. I have been upgrading it a few times each year. I have added Aggregator, Hines Weather, and Xmod during the time too.

I have other similar errors after this 4.9.1 upgrade, which I am going to post as seperate threads so they help other people find the specific solutions. I wonder if they are solved as easily by just removing old DLL files.