It seems the qmake generated sln file does not conform and is not recognized as a Visual Studio 9.00 (2005) solution file.

Replace:
msvc_vcproj.cpp,271: const char _slnHeader80[] = "Microsoft Visual
Studio Solution File, Format Version 9.00";

with
const char _slnHeader80[] = "Microsoft Visual Studio Solution File,
Format Version 9.00\n# Visual Studio 2005";

The added line is just a comment, but important if we want the file to be recognized as a Visual 9.0 file, and opened by the appropriate Visual Studio version.

This issue also applies to more recent Visual Studio versions.