When opening a .pro file as an argument (i.e QtCreator.exe "D:\file.pro") the following error occurs:
Cannot open <filepath>'<filepath>\file.pro' for reading: The filename, directory name, or volume label syntax is incorrect.
I think the problem is with the duplicate <filepath>, it should be instead:
'<filepath>\file.pro' without the <filepath> before.
When opening a .pro file using "Open File or Project" from inside Qt Creator, it works without any problems.
P.S I think this is a very lame bug that should not be in such a big project (Qt), even in ALPHA or BETA stages.