Everything was working fine until this update
Almost certainly because at some point you built your project when all of the code was still in main.cpp, it created the main.moc file, and that file was still hanging around.

What is happening in this case seems to be that either MOC is not processing the header file where you have defined the DownloadManager class, 2) the cpp file generated by the MOC compiler is not being compiled by C++, or 3) the obj file produced by the compiler is not being linked into the executable.

Make sure that your Qt Creator .pro file contains the name of your DownloadManager class header file in the HEADERS line, and then run qmake again before rebuilding.