Problem with SQL Server 2005 Backup Job Maintenance Plan - Error Using Proxy Account

I created a SQL Server Maintenance plan to backup all our databases and store the files on a share.  The job owner is "SA" but the job step is set to "run as" a proxy account.  The proxy account's credential is a domain account that has the following database rights to MSDB - db_backuperator, db_dtsoperator, SQLAgentUserRole.

Now we obviously don't want to give the credential/domain account SysAdmin rights to the server (which BTW, allows this job to run without a hitch).  We just want to give the bare minimum, as per best practice.  So when we run the maintenance plan, the job fails and we get the following error message:

Message
Executed as user: XYZ\sqlbackupadmin. ...9.00.5000.00 for 32-bit  Copyright (C) Microsoft Corp 1984-2005. All rights reserved.    Started:  6:00:00 PM  Error: 2011-03-31 18:00:11.60     Code: 0xC002F210     Source: {2012CDE4-7671-40B1-92F2-26A84187DCDA} Execute SQL Task     Description: Executing the query "DECLARE @Guid UNIQUEIDENTIFIER      EXECUTE msdb..sp_maintplan_open_logentry '{1025AEF1-AC7C-48FB-9D44-C595BAF4CEBC}', '{845EB87D-0CB6-4E83-BFBD-298EA1B60FBD}',NULL, @Guid OUTPUT      Select CONVERT(nvarchar(38),@Guid) AS RunId" failed with the following error: "The EXECUTE permission was denied on the object 'sp_maintplan_open_logentry', database 'msdb', schema 'dbo'.". Possible failure reasons: Problems with the query, "ResultSet" property not set correctly, parameters not set correctly, or connection not established correctly.  End Error  Error: 2011-03-31 18:00:11.72     Code: 0xC0024104     Source: {E8DD7625-07DD-48C0-A98F-DE163EAE8A61}      Descript...  The package execution fa...  The step failed.

I've tried a variety of things like making the domain account (credential) db_owner for MSDB without any success.  Again, the only thing that seems to work is if I give the domain account (credential) SysAdmin rights which won't cut it for us.  Please advise.  I'm at the end of my rope with this...

Thanks in advance to all who reply!