tcprojx project file not found

I'm getting a strange error while running my build:

C:\TeamCity\buildAgent\work\afb48a0fad70f24e\Project1.build.proj(889,  5): error MSB3202: The project file  "C:\TeamCity\buildAgent\work\afb48a0fad70f24e\Project1.build.proj.teamcity.patch.tcprojx"  was not found.

This is an msbuild project, and is happening on the following:

<Target Name="SignExe">
        <ItemGroup>
            <SignFiles Remove="@(SignFiles)" /> <!-- clean existing items -->
            <SignFiles Include="**\*.exe">
        </ItemGroup>

    <PropertyGroup>
      <SignToolFiles>&quot;@(SignFiles, '&quot; &quot;')&quot;</SignToolFiles>
      <BypassTimestamp Condition="'$(Configuration)'=='Debug'">1</BypassTimestamp>
    </PropertyGroup>
    <MsBuild Projects="$(MSBuildProjectFullPath)" Targets="SignToolTask" Properties="BypassTimestamp=$(BypassTimestamp);SignToolFiles=$(SignToolFiles)" />
</Target>

This does work fine in my current CruiseControl server (or if I run by hand). Basically I'm just calling msbuild again on the project file itself to simplify the way parameters are passed -- I don't remember exactly the reason it was set up this way, it may have just been to avoid having global variables. Anyway, the task runs signtool.exe on all the executables.

I can't figure out why this doesn't work:

• Apparently TC is making a .teamcity.patch.tcprojx file -- but I don't see it in the directory while the build is running
• The error output says Project1.build.proj, and yet, the MSBuildProjectFullPath variable is the tcprojx file -- this seems very strange to me.

Any ideas on what can be done to fix this? I'm not necessarily against changing the way I invoke the SignToolTask target, but that seems like it's just ignoring/working around what could be the real issue here.