Well, I tested it a bit longer and it looks like the GIVEN .XML file was not OK.
I couldn't open it, but I could import it to an empty project I created.
Then I saved it as .XML file and then - tadaaamm - I could open it!
So - this issue is no more an issue
Now assume that I opened successfully saved .XML file. I made some minor changes and tried to EXPORT it to MS Project .XML file.
[Project->Export->MS Project button (.XML)->Browse to target directory->Enter path or folder name]
- in "Files" widow I SEE NO .XML FILES (which are there actually) - window is empty
- I enter the filename SOMETHING.XML - and nothing happens (literally).
But maybe I misunderstand the meaning of "export"? Could you comment?