xBuild on Mono on Windows uses different directories for the target files than xBuild on .NET on Windows. There's code elsewhere that redirects C:\Windows\Microsoft.NET to the mono install dir when running on Mono, and does not on .NET. This particular piece of code didn't have the paths right, but up until the target files generated by VS2012 it didn't really matter. Now it does because the target files used by VS2013 projects use different paths.