Errro : This request has probably been tampered with. Close the browser and try again.

Hi,

Details are below

1. Upgraded CMS 6R2 to CMS 7.19.
2. Trying to access admin mode to run page type builder plugin.
3. Removed all restrictions from config to get direct login. But after trying to do any operation, app get broke. see error details.

"This request has probably been tampered with. Close the browser and try again.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: This request has probably been tampered with. Close the browser and try again.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[InvalidOperationException: This request has probably been tampered with. Close the browser and try again.]
   EPiServer.Framework.Web.AspNetAntiForgery.ThrowForgeryException() +295
   EPiServer.Framework.Web.AspNetAntiForgery.ValidateField() +68
   EPiServer.Web.PageExtensions.AntiForgeryValidation.PreInit(Object sender, EventArgs e) +61
   System.EventHandler.Invoke(Object sender, EventArgs e) +0
   System.Web.UI.Page.OnPreInit(EventArgs e) +91
   EPiServer.Shell.WebForms.WebFormsBase.OnPreInit(EventArgs e) +13
   System.Web.UI.Page.PerformPreInit() +21
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1040

"

4. Felt that  FormAuth/login cookie issue, hence  we have creaed one admin account and try to login , but user reamain on login and login cookie get overwriiten or doesn't get created.

5. Tried to disable AnitForgery validation by setting  "requireSSL=false" from config, but no success.

Please sugget, what could be the error?

Thank you