Comment 2 for bug 686091

Okay, it's pretty clear what's going on: update-manager fails to open the meta-release file, so it presumes it's being run for the first time and attempts to create ~/.cache/update-manager-core, which it can't. Could the program fall back to a system-wide file (e.g. under /etc) in a case like this?

(It seems like u-m would also have this problem if the ~/.cache directory doesn't exist for whatever reason, since os.mkdir() doesn't behave like "mkdir -p"...)