So while I was trying to configure a managed service account in SharePoint 2010 today I came across the error “object reference not set to an instance of an object” when I tried accessing the Configure Managed Accounts page (/_admin/ManagedAccounts.aspx). After some research it turns out that the likely culprit here is enabling the automatic password reset on an account that is running in an Active Directory Environment that is set for Pre-Windows 2000 Compatible Access. The automatic password reset feature is only available if your Active Directory environment is running is 2008 mode.
To resolve the error you need to open up the SharePoint Management Console (Powershell) and run the following command against your Farm Account.
$farmAccount = Get-SPManagedAccount domain\loginname $farmAccount.AutomaticChange= $false $farmAccountt.Update()