The Credentials will need to be updated on the SQL server that runs the jobs under the following section:  Security -> Credentials - Refresh ->

Also, the Impersonation information may have to be updated if the user account for the Refresh job changes.  The Impersonation information is located on the Fundraising OLAP database on the Analysis Server.  The data source may need to be changed to ''Use the service account''

More information can be found here:
Unable to start execution of step 1 (reason: Error authenticating proxy [domain\user_name], system error: Logon failure: unknown user name or bad password.). The step failed
How to: Create a Proxy


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