Hi, 

I am unable to create Web application , after farm configuration i can able to create 2 , 3 web applications without any issue but after that i am unable to create any more web applications or site collections. we did not change any thing manually in server.below are the log messages related to the issue.

layouts.sitemap file is missing for all problematic web applications in the below folder. 

C:\inetpub\wwwroot\wss\VirtualDirectories\[port]\_app_bin .

below are the error messages.

Application error when access /_admin/extendvs.aspx, Error=Object reference not set to an instance of an object.  
 at Microsoft.SharePoint.Administration.SPAspSiteMapFile.MergeAspSiteMapFiles(XmlDocument xmldocSiteMap, String strSrcFilePath, String strMergeFilePattern)    
 at Microsoft.SharePoint.Administration.SPAspSiteMapFile.Copy(String strSrcDir, String strSrcDirPrevious, String strSrcLeaf, String strDestDir, Boolean bMergeMaps, Boolean bBackupExistingFile)    
 at Microsoft.SharePoint.Administration.SPWebApplication.CopyAppDomainDirectories(DirectoryInfo vrPath, OverwriteSetting overwrite)    
 at Microsoft.SharePoint.Administration.SPWebApplication.Provision()    
 at Microsoft.SharePoint.ApplicationPages.ExtendVirtualServerPage.BtnSubmit_Click(Object sender, EventArgs e)    
 at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)    
 at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

---------------------------------

System.NullReferenceException: Object reference not set to an instance of an object.   
 at Microsoft.SharePoint.Administration.SPAspSiteMapFile.MergeAspSiteMapFiles(XmlDocument xmldocSiteMap, String strSrcFilePath, String strMergeFilePattern)    
 at Microsoft.SharePoint.Administration.SPAspSiteMapFile.Copy(String strSrcDir, String strSrcDirPrevious, String strSrcLeaf, String strDestDir, Boolean bMergeMaps, Boolean bBackupExistingFile)    
 at Microsoft.SharePoint.Administration.SPWebApplication.CopyAppDomainDirectories(DirectoryInfo vrPath, OverwriteSetting overwrite)    
 at Microsoft.SharePoint.Administration.SPWebApplication.Provision()    
 at Microsoft.SharePoint.ApplicationPages.ExtendVirtualServerPage.BtnSubmit_Click(Object sender, EventArgs e)    
 at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)    
 at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Please let me know how to resolve these issues.