I'm having similar problems. I'm using an n810 and just installed the newest OS update (2.2007.51-3). I've tried installing several different applications and none will download. An example of the error log is below. I tried reinstalling the OS update but problem persists. I have plenty of available memory on the device, and plenty on each individual memory slot/card. Please advise as this is extremely frustrating.

hildon-application-manager 2.0.2
apt-worker command 3 already pending
apt-worker: domain change: map (nokia-system -> unsigned)
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Installing mnotify 2.0.3
Failed to fetch http://catalogue.tableteer.nokia.com...0.3_armel.deb: Could not open file /media/mmc2/.apt-archive-cache/partial/mnotify_2.0.3_armel.deb - open (30 Read-only file system) [IP: 72.246.19.41 80]
W: Not using locking for read only lock file /media/mmc2/.apt-archive-cache/lock
W: Not using locking for read only lock file /media/mmc2/.apt-archive-cache/lock