I remember having this problem some time ago after updating Qt version.
I'm not sure this was the only thing needed but make sure to remove the entire build directory and rebuild your project.
If that doesn't help you might try this (possibly for each file that has Q_OBJECT macro):
remove (don't just comment out, remove entirely) the Q_OBJECT macro
save the file (this forces Qt plugin to remove generated files configuration for that file)
add the Q_OBJECT macro back
save the file (this will force Qt plugin to regenerate moc information for that file)
Try this with one file and if it helps repeat with others.