[Qt-creator] ui_files not found, do not compile

[Matthias Pospiech]

After I saved the project, and the .pro file was recreated from 
QtCreator my ui Files do not compile anymore.
In the editor they are not found and the compiler complains:

mingw32-make[1]: *** No rule to make target `ui_mainwindow.h', needed by 
`build/release/mainwindow.o'. Stop.


The ui files are place in folder 'ui', according to "UI_DIR = ui"
They are existend in that folde as expected.

The include Path is also extendet with this foldes:
INCLUDEPATH = src;ui;

However the error from above happens.

Any ideas?