Error Authenticating Proxy Account when Executing SSIS job

I have a SQL Server instance that runs 5 scheduled tasks each night, each of which run SSIS packages. These packages have been running for years and the associated steps run via a Proxy Account (PackageExecutor). PackageExecuter is associated with a SQL Credential that was a former domain admin account. Soon, the domain associated with this admin account is going to be shutdown. I have to use a new account, on a new domain, as the admin account associated with my proxy, PackageExecutor. When I created a new Credential for the new Admin account and associated it with PackageExecutor, I started to get the following error when I tried to run one of my SQL jobs as a test: Unable to start execution of step 1 (reason: Error authenticating proxy *Domain\Admin_Account*@*fully.qualified.domain.com*, system error: Logon failure: unknown user name or bad password.). The step failed. If I understand this reasonably explicit error, what it's telling me is that the Credential accounts, associated with my proxy is not correct. How do I validate this? I know that this account is legitimate-- I've already associated it with every associated server group, I have made it a sysadmin user on the server. What could be causing this problem? To be clear, I haven't mis-typed the account name or the password associated with the Proxy Credential. However, when I entered the account name Domain\Admin_Account and clicked the Check Names button, SQL Server automatically transformed the User ID to the fully-qualified version. I'm not sure if this has anything to do with this problem. I'm at a bit of a loss. I've given my credential account full access to everything that I can think of. What might I need to do to get this to work? **UPDATE** Sorry, one more quick mention. I've found this MSDN kb article . Resolution method #1 is what I've been doing for years. The others don't seem to apply, or I'm missing something. Any tips or clarification would be beneficial.