errors during Execute Process Task validation

I am using an Execute Process task in SSIS (SQL Server 2005). I am supplying the location of the executable and the arguements via package variables and expressions.

the package variables are loaded like this:
...
ElseIf Dts.Variables.Item("User::DEV_SERVER_CONSTANT").Value.ToString = Dts.Variables.Item("System::MachineName").Value.ToString Then
            Dts.Variables.Item("User::strDEPCmd").Value = Dts.Variables.Item("User::MYDEP_EXE_DEV").Value.ToString
            Dts.Variables.Item("User::strDEPCmdArgs").Value = " server=" + Dts.Variables.Item("User::strSQLServer").Value.ToString + ";USER=AUTO;PWD=;USENT=Y;"
        ElseIf ...

MSGBOXing the variables out yields these results:
User::DEV_SERVER_CONSTANT = ComputerName
Both User::MYDEP_EXE_DEV and User::strDEPCmd = \\ComputerName\SharedFolder\SSIS_TEST\Mydep.exe
User::strDEPCmdArgs =<space>server=ComputerName\Instance;USER=AUTO;PWD=;USENT=Y;
User::strSQLServer = ComputerName\Instance

I created an expression in the Execute Process task for the executable that looks like this:
@[User::strDEPCmd] + @[User::strDEPCmdArgs]

Everything seems to be going smoothly on debug but I then get the following error:
...
Error: 0xC0029153 at Run MyDeposit, Execute Process Task: File/Process "Instance;USER=AUTO;PWD=;USENT=Y;" does not exist in directory "\\ComputerName\SharedFolder\SSIS_TEST\mydep.exe server=ComputerName".
Error: 0xC0024107 at Run MyDeposit: There were errors during task validation...

I have attempted to supply these values separately, as well. In that case, I removed the <space> from User::strDEPCmdArgs and supplied it in the box for arguments. This yielded the following result:

Error Desc:     [DBNETLIB][ConnectionOpen (Invalid Instance()).]Invalid connection.
Error Code:     -2147467259:80004005
Error Source:   modmain.main
Error System:   Microsoft
Error Location: Line: 0
Error Suggested Response: 0Start Free Trial