The problem occurred somewhere in the following section of the log file. I'm afraid I don't understand the log file format enough to narrow down in which section the relevant event occurred:

Wed 2011-12-07 07:51:33 +0000
0.057  bazaar version: 2.4.0
0.057  bzr arguments: [u'qswitch', u'--ui-mode']
0.089  looking for plugins in F:\VCS\Utilities\Plugins
0.374  looking for plugins in C:\Windows\system32\config\systemprofile\Application Data\bazaar\2.0\plugins
0.374  looking for plugins in C:/Program Files/Bazaar/plugins
0.375  Plugin name explorer already loaded
0.376  Plugin name qbzr already loaded
0.461  encoding stdout as osutils.get_user_encoding() 'cp1252'

Wed 2011-12-07 07:52:00 +0000
0.058  bazaar version: 2.4.0
0.059  bzr arguments: [u'qsubprocess', u'--bencode', u'l6:switch61:F:\\VCS\\LibraryModules\\CommonProtocolStack\\WriteFileForwardinge']
0.091  looking for plugins in F:\VCS\Utilities\Plugins
0.368  looking for plugins in C:\Windows\system32\config\systemprofile\Application Data\bazaar\2.0\plugins
0.369  looking for plugins in C:/Program Files/Bazaar/plugins
0.369  Plugin name explorer already loaded
0.370  Plugin name qbzr already loaded
0.451  encoding stdout as osutils.get_user_encoding() 'cp1252'
0.456  bazaar version: 2.4.0
0.456  bzr arguments: [u'switch', u'F:\\VCS\\LibraryModules\\CommonProtocolStack\\WriteFileForwarding']
0.460  encoding stdout as osutils.get_user_encoding() 'cp1252'
0.503  opening working tree 'C:/jon/jm_bzr/CommonProtocolStack'
0.855  Transferred: 0kB (0.0kB/s r:0kB w:0kB)
0.857  encoding stdout as osutils.get_user_encoding() 'cp1252'
0.952  opening working tree 'C:/jon/jm_bzr/CommonProtocolStack'
1.065  Using fetch logic to copy between CHKInventoryRepository('file:///F:/VCS/LibraryModules/CommonProtocolStack/.bzr/repository/')(RepositoryFormat2a()) and CHKInventoryRepository('file:///C:/jon/jm_bzr/CommonProtocolStack/.bzr/repository/')(RepositoryFormat2a())
1.066  fetching: <SearchResult search:(set(['rob.umfreville@<redacted>.com-20111206153305-zrg0jo1v7nphcua6']), ['rob.umfreville@<redacted>.com-20111025150514-4vcrh3v1u9k85xoy', 'jon.mills@<redacted>.com-20111020090747-ckovf2274w56kqa4', 'jon.mills@<redacted>.com-20111102144004-9pqh6n9lefrdauaf', 'rob.umfreville@<redacted>.com-20111025154323-4vb4dsx9e15xkg6w', 'rob.umfreville@<redacted>.com-20111014144516-zkd04soo9y0xt8p6', ...], 4)>
149.624  check paths: None
149.696  check paths: None
149.746  check paths: None
149.793  check paths: None
149.844  check paths: None
149.911  check paths: None
149.957  check paths: None
150.004  check paths: None
150.055  check paths: None
150.102  check paths: None
150.150  check paths: None
150.196  check paths: None
150.242  check paths: None
1.960  Deprecated function called
Called from:
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 157, in pull
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 61, in read_config
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 50, in _set_config
1.960  Deprecated function called
Called from:
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 157, in pull
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 61, in read_config
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 50, in _set_config
1.960  Deprecated function called
Called from:
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 157, in pull
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 61, in read_config
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 50, in _set_config
1.980  Deprecated function called
Called from:
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 158, in pull
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 82, in read_config_from_repository
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 50, in _set_config
1.980  Deprecated function called
Called from:
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 158, in pull
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 82, in read_config_from_repository
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 50, in _set_config
1.981  Deprecated function called
Called from:
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 158, in pull
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 82, in read_config_from_repository
  File "F:\VCS\Utilities\Plugins\externals\externals.py", line 50, in _set_config
[ 2772] 2011-12-07 07:52:02.805 INFO: External pull F:/VCS/LibraryModules/CircularBuffer/trunk --directory Testing/gtest/LibraryModules/CircularBuffer
1.984  bazaar version: 2.4.0
1.984  bzr arguments: [u'pull', u'F:/VCS/LibraryModules/CircularBuffer/trunk', u'--directory', u'Testing/gtest/LibraryModules/CircularBuffer']
1.988  encoding stdout as osutils.get_user_encoding() 'cp1252'
1.994  opening working tree 'C:/jon/jm_bzr/CommonProtocolStack/Testing/gtest/LibraryModules/CircularBuffer'
2.094  Using fetch logic to copy between CHKInventoryRepository('file:///F:/VCS/LibraryModules/CircularBuffer/.bzr/repository/')(RepositoryFormat2a()) and CHKInventoryRepository('file:///C:/jon/jm_bzr/CommonProtocolStack/Testing/gtest/LibraryModules/CircularBuffer/.bzr/repository/')(RepositoryFormat2a())
2.094  fetching: <SearchResult search:(set(['alan.budden@<redacted>.com-20111007124335-l29c6ugt06osavvj']), ['jon.mills@<redacted>.com-20110527170751-slsqxs208g4r40fs', 'alan.budden@<redacted>.com-20110602142421-aeltbufds6zylzef', 'alan.budden@<redacted>.com-20110519143035-qyatwsuumcpje9zn', 'alan.budden@<redacted>.com-20110519100626-0cf0fwenszg72dc1', 'alan.budden@<redacted>.com-20110602133644-m0ulwh07x2uxka2z', ...], 0)>
2.110  Transferred: 0kB (0.0kB/s r:0kB w:0kB)
[ 2772] 2011-12-07 07:52:02.933 INFO: External pull F:/VCS/LibraryModules/CMSIS_STM32/trunk --directory Testing/gtest/LibraryModules/CMSIS
2.111  bazaar version: 2.4.0
2.111  bzr arguments: [u'pull', u'F:/VCS/LibraryModules/CMSIS_STM32/trunk', u'--directory', u'Testing/gtest/LibraryModules/CMSIS']
2.115  encoding stdout as osutils.get_user_encoding() 'cp1252'
150.433  None

150.433  Traceback (most recent call last):
  File "C:/Program Files/Bazaar/plugins\explorer\lib\view_workingtree.py", line 142, in refresh_view
  File "C:/Program Files/Bazaar/plugins\explorer\lib\view_workingtree.py", line 116, in _add_file_watcher_paths
  File "bzrlib\osutils.pyo", line 1772, in _walkdirs_utf8
  File "_walkdirs_win32.pyx", line 227, in bzrlib._walkdirs_win32.Win32ReadDir.read_dir (bzrlib\_walkdirs_win32.c:1837)
WindowsError: [Error 5] C:/jon/jm_bzr/CommonProtocolStack/Testing/gtest/LibraryModules/CircularBuffer/.bzr/branch/lock/releasing.bkc3z1yiuz9p6zwod0k4.tmp/*

150.434  No Apport available to Bazaar
2.120  opening working tree 'C:/jon/jm_bzr/CommonProtocolStack/Testing/gtest/LibraryModules/CMSIS'
2.197  Using fetch logic to copy between CHKInventoryRepository('file:///F:/VCS/LibraryModules/CMSIS_STM32/.bzr/repository/')(RepositoryFormat2a()) and CHKInventoryRepository('file:///C:/jon/jm_bzr/CommonProtocolStack/Testing/gtest/LibraryModules/CMSIS/.bzr/repository/')(RepositoryFormat2a())
2.197  fetching: <SearchResult search:(set(['rob.umfreville@<redacted>.com-20111110152441-29ejpyr0it8o0cq0']), ['alan.budden@<redacted>.com-20110810085205-rm4yo2br973k5tjj', 'alan.budden@<redacted>.com-20110117094118-zjxjikc127ctm2y4', '<email address hidden>', '<email address hidden>', 'alan.budden@<redacted>.com-20100223103936-pupvilj8l8isw7le', ...], 0)>
2.213  Transferred: 0kB (0.0kB/s r:0kB w:0kB)
[ 2772] 2011-12-07 07:52:03.036 INFO: External pull F:/VCS/LibraryModules/RotorkDefinitions/trunk --directory Testing/gtest/LibraryModules/RotorkDefinitions
2.214  bazaar version: 2.4.0
2.214  bzr arguments: [u'pull', u'F:/VCS/LibraryModules/RotorkDefinitions/trunk', u'--directory', u'Testing/gtest/LibraryModules/RotorkDefinitions']
2.218  encoding stdout as osutils.get_user_encoding() 'cp1252'
2.229  opening working tree 'C:/jon/jm_bzr/CommonProtocolStack/Testing/gtest/LibraryModules/RotorkDefinitions'
2.295  Using fetch logic to copy between CHKInventoryRepository('file:///F:/VCS/LibraryModules/RotorkDefinitions/.bzr/repository/')(RepositoryFormat2a()) and CHKInventoryRepository('file:///C:/jon/jm_bzr/CommonProtocolStack/Testing/gtest/LibraryModules/RotorkDefinitions/.bzr/repository/')(RepositoryFormat2a())
2.295  fetching: <SearchResult search:(set(['simon.nottingham@<redacted>.co.uk-20111028084357-en2129z1qn3gxf6q']), ['jon.mills@<redacted>.com-20100802094343-uewmzse0c6k6q6un', 'jon.mills@<redacted>.com-20100812162812-56jsejenxqztt6tr', 'simon.nottingham@<redacted>.co.uk-20111028084357-en2129z1qn3gxf6q', 'alan.budden@<redacted>.com-20100311121737-dxy4di2wtbbm01ra', 'alan.budden@<redacted>.com-20100311124216-ya0ttd1st9ev42s0', ...], 0)>
2.311  Transferred: 0kB (0.0kB/s r:0kB w:0kB)
2.387  opening working tree 'C:/jon/jm_bzr/CommonProtocolStack'
[ 2772] 2011-12-07 07:52:04.295 INFO: Updated to revision 141.
[ 2772] 2011-12-07 07:52:04.394 INFO: Switched to branch: F:/VCS/LibraryModules/CommonProtocolStack/WriteFileForwarding/
3.573  Transferred: 0kB (0.0kB/s r:0kB w:0kB)
3.573  Transferred: 0kB (0.0kB/s r:0kB w:0kB)
3.573  return code 0
35.497  return code 0
285.635  return code 0

Wed 2011-12-07 07:54:21 +0000
0.057  bazaar version: 2.4.0
0.057  bzr arguments: [u'explorer']
0.090  looking for plugins in F:\VCS\Utilities\Plugins
0.368  looking for plugins in C:\Windows\system32\config\systemprofile\Application Data\bazaar\2.0\plugins
0.368  looking for plugins in C:/Program Files/Bazaar/plugins
0.369  Plugin name explorer already loaded
0.370  Plugin name qbzr already loaded
0.460  encoding stdout as osutils.get_user_encoding() 'cp1252'
0.609  Deprecated method called
Called from:
  File "C:/Program Files/Bazaar/plugins\explorer\lib\explorer.py", line 165, in __init__
  File "C:/Program Files/Bazaar/plugins\explorer\lib\explorer.py", line 257, in _refresh_user_configuration
  File "C:/Program Files/Bazaar/plugins\explorer\lib\explorer.py", line 264, in get_default_editor
0.611  loading explorer extensions for clothes [u'Bazaar support', u'Register on Launchpad']
0.612  explorer extensions provided by the colo plugin in C:/Program Files/Bazaar/plugins/colo/explorer
0.618  explorer extensions provided by the rename_reference_project plugin in F:/VCS/Utilities/Plugins/rename_reference_project/explorer
0.621  explorer extensions provided by the source_creator plugin in F:/VCS/Utilities/Plugins/source_creator/explorer
0.695  setting preference show-extended-checkout-title to true (was true)
0.695  setting preference toolbar-contents to expanded (was expanded)
0.696  setting preference show-extended-bound-branch-title to true (was true)
0.696  setting preference workspace-model to remote-feature-branches (was remote-feature-branches)
0.696  setting preference language to en_GB (was en_GB)
0.696  setting preference bind-branches-by-default to True (was True)
0.696  setting preference status-auto-refresh to False (was False)
0.696  setting preference toolbox-on-status-view to False (was False)
0.696  setting preference toolbox-style to tree (was tree)
59.049  opening working tree 'C:/jon/jm_bzr/CommonProtocolStack'
59.227  check paths: None
59.265  check paths: None