Satellite 5.3 and older: "TypeError: unsubscriptable object" when importing channel dumps or inter-satellite-sync via satellite-sync

Issue

• satellite-sync fails with the following error message while importing a channel from a channel dump generated via 5.4 Satellite server.
• satellite-sync fails with the following error message while importing a channel dump downloaded from RHN (rhn.redhat.com).
• satellite-sync fails with the following error message while importing a channel during Inter Satellite Sync.

Exception Handler Information
Traceback (most recent call last):
  File "/usr/bin/satellite-sync", line 142, in main
    return satsync.Runner().main()
  File "/usr/share/rhn/satellite_tools/satsync.py", line 212, in main
    ret = method()
  File "/usr/share/rhn/satellite_tools/satsync.py", line 324, in _step_packages
    self._affected_channels = self.syncer.import_packages()
  File "/usr/share/rhn/satellite_tools/satsync.py", line 1653, in import_packages
    sync_handlers.import_packages(batch)
  File "/usr/share/rhn/satellite_tools/sync_handlers.py", line 434, in import_packages
    importer.run()
  File "/usr/share/rhn/server/importlib/importLib.py", line 628, in run
    self.submit()
  File "/usr/share/rhn/server/importlib/packageImport.py", line 308, in submit
    transactional=self.transactional)
  File "/usr/share/rhn/server/importlib/backend.py", line 741, in processPackages
    transactional=transactional)
  File "/usr/share/rhn/server/importlib/backend.py", line 1521, in __processObjectCollection__
    _buildExternalValue(extObject, entry, tbl)
  File "/usr/share/rhn/server/importlib/backend.py", line 2018, in _buildExternalValue
    dict[f] = sanitizeValue(entry[attr], datatype)
  File "/usr/share/rhn/server/importlib/backendLib.py", line 469, in sanitizeValue
    return value[:datatype.limit]
TypeError: unsubscriptable object