I can open c:\Users\Mapmuh\Pictures\20130401\file.jpg with a double click
but I can't open c:\Documents and Settings\Marcin\Pictures\20130401\file.jpg (which strangely is the same file

Moreover I can open artificially created c:\Users\Marcin\Pictures\DOCUMENTS AND SETTINGS\20130401\file.jpg

Hello, strange issue - difficult to replicate.
XNViewMP after a reboot doesn't open files from file manager.
I have my pictures in C:\Users\Mapmuh\Pictures\20130616 (and similarly named yearmonthday catalogues). After double clicking a jpg file, XNViewMP opens up the main window without an image. But if I first open an image located in C:\ (& closing XNView) then an image located in C:\Users\ (& closing XNView) suddenly everythings gets fine and I can open files from whichever they are (even with spaces in filename, which I suspected to be the cause of the problem at first). Even if I turn the programme off.
Issue was just the same in 0.53 when I noticed it for the first time.
PS. Forgot to mention: file associations were done thru "Associate files with XnViewMP" app.