Comment 18 for bug 688682

This bug is fixed in the current code, however its not fully able to deal with the breakage it caused earlier.

The bug itself got fixed in 3.1.25:

--- software-center-3.1.23.1/softwarecenter/db/update.py 2011-03-07 10:15:24.000000000 +0100
+++ software-center-3.1.26.6/softwarecenter/db/update.py 2011-04-14 13:53:17.000000000 +0200
@@ -30,7 +30,6 @@
 import time
 import urllib
 import xapian
-import softwarecenter.log

 from ConfigParser import RawConfigParser, NoOptionError
 from gettext import gettext as _

The import had caused a subsequent import for softwarecenter.paths and that has created the dir. There is a check
there for os.getuid() == 0. Then HOME is unset, but xdg.BaseDirectory.xdg_cache_home was already created so
the updated HOME was not taken into account.