Okular source references don't work on Windows at the moment when synctex is involved (compiled with MSVC 2017).
The problem seems to originate from the fact that the wchar_t versions of PathFindFileName and PathFindExtension are automatically used.
However, path names are given in an 8-bit encoding to synctex (QFile::encodeName) and the synctex code is really only prepared for 8-bit encodings.