I am trying to connect my n900 to my computer via a usb cable. It used to work. Now, when I connect the cable and select USB Mass Storage, it will tell me that the memory of the device is in use (or what is the proper translation, the phone speaks to me in Czech).

I tried to unmount /home/user/MyDocs, but it tells me that "the device or resource is busy" - however, when I try to find out why, lsof wont give me an answer. Any ideas what might be wrong?

Don't try to access the device, from the device, when it's plugged it.
The memory, or more precisely the MyDocs storage "is" in use, by the pc! Remount MyDocs and everything should go back to normal.
The computer's file manager should be able to "see" the FAT32 sections of the device while plugged it.

yeah I know, the problem was that it was mounted, not that it was not mounted.

Anyway,

umount -l /home/usr/MyDocs

worked