New Post: The OutputPath property is undefined

When running MSIs from a script we've recently started to encounter an error: "error : Cannot continue. The OutputPath property is undefined."

The same msi installs fine when it is run from the command line.

The command line that the script executes is:

C:\Windows\Microsoft.NET\Framework\v4.0.30319\MSBuild.exe /p:DeployBizTalkMgmtDB=False;Configuration=Server;SkipUndeploy=true "C:\Program Files (x86)\xxx\yyy.btdfproj"

(Where xxx and yyy point to the appropriate btdfproj file).

Our btdfproj file contains the following:

<PropertyGroup Condition="'$(Configuration)' == 'Debug'">
    <DeploymentFrameworkTargetsPath>$(MSBuildExtensionsPath)\DeploymentFrameworkForBizTalk\5.0\</DeploymentFrameworkTargetsPath>
    <OutputPath Condition="'$(TeamBuildOutDir)' == ''">bin\Debug\</OutputPath>
    <OutputPath Condition="'$(TeamBuildOutDir)' != ''">$(TeamBuildOutDir)</OutputPath>
    <DeployPDBsToGac>false</DeployPDBsToGac>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)' == 'Release'">
    <DeploymentFrameworkTargetsPath>$(MSBuildExtensionsPath)\DeploymentFrameworkForBizTalk\5.0\</DeploymentFrameworkTargetsPath>
    <OutputPath Condition="'$(TeamBuildOutDir)' == ''">bin\Release\</OutputPath>
    <OutputPath Condition="'$(TeamBuildOutDir)' != ''">$(TeamBuildOutDir)</OutputPath>
    <DeployPDBsToGac>false</DeployPDBsToGac>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)' == 'Server'">
    <DeploymentFrameworkTargetsPath>Framework\</DeploymentFrameworkTargetsPath>
    <!-- Get our PDBs into the GAC so we get file/line number information in stack traces. -->
    <DeployPDBsToGac>true</DeployPDBsToGac>
  </PropertyGroup>

We've recently upgraded our installation of BTDF from v5.0 RC2 to v5.0 RTW, and started to create the MSIs on our build server. Prior to these changes, the MSIs installed without issue.

I would appreciate any suggestions as to how I can diagnose the issue and fix our process.