After connecting N9 to a Linux machine, any other machine is unable to connect to it. Windows XP, W7 and another Linux machine can see N9 and Windows assigns it a drive letter even, but it seems like the filesystem is not recognized anymore and the contents can't be seen. I'm setting the connection type to mass memory mode on the device.
The problem is that currently I don't have access to the Linux machine where the connection worked effortlessly.
SOLVED: hard reset (10 sec powerbutton) seems to have fixed the problem.