USB and streaming strange issue

[Sytten]

Hi,

Not sure I posted in the right section but anyway.
I have a really strange issue.
I stream my videos from my NAS over wifi with a cheap usb dongle.
It works great ONLY when the second port on my RPi is connected to something like a keyboard, otherwise I can't open the file and I get an error from XBMC (basically it tells me to go in the log file where I found that the movie couldn't be open...)

Do you have any idea what is the problem?

Thank you
Sytten