The report definition is not valid. Details: The report definition has an invalid target namespace 'http://schemas.microsoft.com/sqlserver/reporting/2008/01/reportdefinition' which cannot be upgraded.
Turns out the issue was my "C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v10.0\ReportingServices\Microsoft.ReportingServices.targets" file had somehow changed. The top of my file was:
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<UsingTask TaskName="Microsoft.Reporting.RdlCompile" AssemblyName="Microsoft.ReportViewer.Common, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
And it should have been:
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<UsingTask TaskName="Microsoft.Reporting.RdlCompile" AssemblyName="Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
