msbuild calling powershell script under windows 7 gets signing error
Just guessing: You did run Set-ExecutionPolicy in your x64 PowerShell and the build runs the x86 PowerShell?
Maybe try Get-ExcutionPolicy in both shells.
Edit: oops, I see you already enabled scripts. It's possibly something to do with the context under which MSBuild is running. Are you running MSBuild as a local service account of some kind?
Edit2: It's clear the error message is deceptive. There's something else going on and the execution policy error is being thrown but not for the expected reason. Perhaps as a way to isolate, try creating another MSBuild task that runs the same line but with shorter, hardcoded paths instead of the /../../../ bonanza above?