Delete User alerts in sharepoint giving Access Denied?

I am trying to delete user alerts from _layouts/sitesubs.aspx page login with site collection admin account. It throwing to "Access denied" page.

Logged in user has full permission to the site collection. I am not understanding why this happening.

Please help me out.

Update: I used alert.asmx web service also. It is throwing "Server Error".

Update:

Logs

' 6ovk  Verbose    Console XML successfully loaded from SiteAction on page http://dev.org/_layouts/sitesubs.aspx.    
6ovm    Verbose     End OnLoad of XmlConsoleDataSource from file "SiteAction".   
8ofj    Verbose     Menu::OnLoad() - Was called. ID="SiteActionsMenu"    
8xfr    Verbose     PermissionMask check failed. asking for 0x00000000, have 0x00000040  
8e2s    Medium      Unknown SPRequest error occurred. More information: 0x80070005   
8gs1    Verbose     Access Denied for /TIGS/ALIF/_layouts/sitesubs.aspx. StackTrace: Microsoft.SharePoint.Utilities.SPUtility:Void HandleAccessDenied(System.Exception), Microsoft.SharePoint.SPGlobal:Void HandleUnauthorizedAccessException(System.UnauthorizedAccessException), Microsoft.SharePoint.Library.SPRequest:Void DeleteSubscription(System.String, System.String, System.String, Boolean, UInt32, Boolean, Int32), Microsoft.SharePoint.SPAlertCollection:Void Delete(System.Guid), Microsoft.SharePoint.ApplicationPages.SubsManageBasePage:Void DeleteAlerts(Microsoft.SharePoint.SPUser, Boolean, System.Web.UI.WebControls.PlaceHolder, System.Web.UI.WebControls.PlaceHolder, System.Web.UI.WebControls.PlaceHolder, System.Web.UI.WebControls.PlaceHolder, System.Web.UI.WebControls.Repeater, System.Web.UI.WebControls.Repeate...  
8gs1    Verbose     ...r, System.Web.UI.WebControls.Repeater), Microsoft.SharePoint.ApplicationPages.SiteSubsPage:Void BtnDeleteAlerts_Click(System.Object, System.EventArgs), Microsoft.SharePoint.WebControls.SPLinkButton:Void OnClick(System.EventArgs), Microsoft.SharePoint.WebControls.SPLinkButton:Void System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(System.String), System.Web.UI.Page:Void RaisePostBackEvent(System.Web.UI.IPostBackEventHandler, System.String), System.Web.UI.Page:Void RaisePostBackEvent(System.Collections.Specialized.NameValueCollection), System.Web.UI.Page:Void ProcessRequestMain(Boolean, Boolean), System.Web.UI.Page:Void ProcessRequest(Boolean, Boolean), System.Web.UI.Page:Void ProcessRequest(), System.Web.UI.Page:Void ProcessRequestWithNoAssert(System.Web.HttpContext), System.Web.UI.Page...   
8gs1    Verbose     ...:Void ProcessRequest(System.Web.HttpContext), ASP._layouts_sitesubs_aspx:Void ProcessRequest(System.Web.HttpContext), System.Web.HttpApplication+CallHandlerExecutionStep:Void System.Web.HttpApplication.IExecutionStep.Execute(), System.Web.HttpApplication:System.Exception ExecuteStep(IExecutionStep, Boolean ByRef), System.Web.HttpApplication+ApplicationStepManager:Void ResumeSteps(System.Exception), System.Web.HttpApplication:System.IAsyncResult System.Web.IHttpAsyncHandler.BeginProcessRequest(System.Web.HttpContext, System.AsyncCallback, System.Object), System.Web.HttpRuntime:Void ProcessRequestInternal(System.Web.HttpWorkerRequest), System.Web.HttpRu开发者_StackOverflow社区ntime:Void ProcessRequestNoDemand(System.Web.HttpWorkerRequest), System.Web.Hosting.ISAPIRuntime:Int32 ProcessRequest(IntPtr, Int32),      
0   Verbose     Releasing SPRequest with allocation Id {0AEEB070-7A62-4D59-A6D9-F5F62BC68F55}    
0   Verbose     Releasing SPRequest with allocation Id {9E124F41-3A29-48BD-ABFB-089EB6EB89C9}    
0   Verbose     Releasing SPRequest with allocation Id {49C107D3-D7FF-47D4-8410-6044593498B9}    
6t8b    Verbose     Looking up context  site http://dev.org:80/TIGS/ALIF/_layouts/AccessDenied.aspx in the farm SharePoint_Config    
6t8d    Verbose     Looking up the additional information about the typical site http://dev.org:80/TIGS/ALIF/_layouts/AccessDenied.aspx.     
6t8f    Verbose     Site lookup is replacing http://dev.org:80/TIGS/ALIF/_layouts/AccessDenied.aspx with the alternate access url http://dev.org.    
6t8g    Verbose     Looking up typical site http://dev.org:80/TIGS/ALIF/_layouts/AccessDenied.aspx in web application SPWebApplication Name=DEV - 80 Parent=SPWebService.    
6t8h    Verbose     Found typical site / (d4771114-7c8d-49fe-b697-7a388e77b501) in web application SPWebApplication Name=DEV - 80 Parent=SPWebService.   
880l    Verbose     ConnectionString: 'Data Source=spdbdev1\dev;Initial Catalog=SharePoint_Config;Integrated Security=True;Enlist=False'    ConnectionState: Closed ConnectionTimeout: 15    
8acb    Verbose     Reverting to process identity    
8acc    Verbose     Current user before SqlConnection.Open: Name: INSIDE\sa_commpooldev SID: S-1-5-21-2651373318-460420032-838475144-1161 ImpersonationLevel: None   
8acf    Verbose     Current user after SqlConnection.Open: Name: INSIDE\sa_commpooldev SID: S-1-5-21-2651373318-460420032-838475144-1161 ImpersonationLevel: None    
880m    Verbose     SqlCommand: 'dbo.proc_getObject'     CommandType: StoredProcedure CommandTimeout: 0     Parameter: '@RETURN_VALUE' Type: Int Size: 0 Direction: ReturnValue Value: ''     Parameter: '@Id' Type: UniqueIdentifier Size: 0 Direction: Input Value: '386869e5-fb7c-4000-b54f-30928bd7ba1b'     
880l    Verbose     ConnectionString: 'Data Source=spdbdev1\spedev;Initial Catalog=SharePoint_Config;Integrated Security=True;Enlist=False'    ConnectionState: Closed ConnectionTimeout: 15     
8acb    Verbose     Reverting to process identity    
8acc    Verbose     Current user before SqlConnection.Open: Name: INSIDE\sa_commpooldev SID: S-1-5-21-2651373318-460420032-838475144-1161 ImpersonationLevel: None   
8acf    Verbose     Current user after SqlConnection.Open: Name: INSIDE\sa_commpooldev SID: S-1-5-21-2651373318-460420032-838475144-1161 ImpersonationLevel: None    
880m    Verbose     SqlCommand: 'dbo.proc_getObject'     CommandType: StoredProcedure CommandTimeout: 0     Parameter: '@RETURN_VALUE' Type: Int Size: 0 Direction: ReturnValue Value: ''     Parameter: '@Id' Type: UniqueIdentifier Size: 0 Direction: Input Value: '45b66929-08b5-4715-8511-f9fb94f614ee'     
880l    Verbose     ConnectionString: 'Data Source=spdbdev1\dev;Initial Catalog=SharePoint_Config;Integrated Security=True;Enlist=False'    ConnectionState: Closed ConnectionTimeout: 15    
8acb    Verbose     Reverting to process identity    
8xfr    Verbose     PermissionMask check failed. asking for 0x00000000, have 0x00000040'     

Do you have any web application policies set that would prevent this user from managing user alerts?

Have a look in Central Administration > Application Management > Policy for Web Application and ensure that you aren't in anything that includes the deny policy against the "Manage Alerts" site permission.