The security validation for this page is invalid

Problem:

After applying SharePoint 2010 Service Pack 1 and June 2011 Update, you start getting an exception “The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.” in your InfoPath forms when you try to do certain actions like adding an attachment to the form.

Checking the ULS logs, you see the following error:

Message : Failed to get SPGroupName from GroupID. Error Message: Group cannot b
e found.  Callstack:    at Microsoft.SharePoint.SPGroupCollection.Get
ByID(Int32 id)     at Microsoft.SharePoint.WebControls.PeopleEditor.s
et_SharePointGroupID(Int32 value).

Cause

This is a known bug in the June 2011 update and there is no known fix from Microsoft at the time this post is written.

Solution

The work-around is to disable Web Page Security Validation  in the Web Application settings.

1. Open Central Administration -> Manage Web Applications -> General Settings
2. Web Page Security Validation -> Off.
3. OK

Note: This work-around causes issues with InfoPath form postbacks and creating document libraries / subsites. Use the following guide instead to fix this issue:

http://www.mysharepointadventures.com/2011/08/june-2011-cu-woes-workaround-for-infopath-forms-error/