I hit a problem where my memory card was set to read only when I flashed the latest OS. I then got a problem each time I tried to install a new app. Could that be your problem? If so I had to unmount (via umount, no first 'n' in there, umount, not unmount, take note) the SD card.

Grab the actual text of your error message and google it, and you'll probably find someone who dealt with, and has advice on how to fix, the exact same error. It was a problem I got on my n800 and not my n810, if I recall correctly.

I'd be more specific, but I didn't take notes, and I don't remember the solution offhand. I could try to drudge it up if you want, PM me if you think you have the same problem, but I think others posted solutions to it, so you may find them first.