This began happening here:
https://ci.chromium.org/buildbot/chromium.infra.cron/wpt-importer/25887

From the end of the logs:

2018-10-08 02:21:22,493 - All jobs finished.
2018-10-08 02:21:22,493 - Adding test expectations lines to LayoutTests/TestExpectations.
2018-10-08 02:21:47,025 - Received HTTP status 400 loading "https://test-results.appspot.com/data/layout_results/mac10_13_retina-blink-rel/723/webkit_layout_tests (with patch)/layout-test-results/failing_results.json".  Retrying in 10 seconds...
2018-10-08 02:21:57,043 - Received HTTP status 400 loading "https://test-results.appspot.com/data/layout_results/mac10_13_retina-blink-rel/723/webkit_layout_tests (with patch)/layout-test-results/failing_results.json".  Retrying in 15.0 seconds...
2018-10-08 02:22:12,076 - Received HTTP status 400 loading "https://test-results.appspot.com/data/layout_results/mac10_13_retina-blink-rel/723/webkit_layout_tests (with patch)/layout-test-results/failing_results.json".  Retrying in 22.5 seconds...
2018-10-08 02:22:34,606 - Received HTTP status 400 loading "https://test-results.appspot.com/data/layout_results/mac10_13_retina-blink-rel/723/webkit_layout_tests (with patch)/layout-test-results/failing_results.json".  Retrying in 33.75 seconds...
2018-10-08 02:23:08,397 - Received HTTP status 400 loading "https://test-results.appspot.com/data/layout_results/mac10_13_retina-blink-rel/723/webkit_layout_tests (with patch)/layout-test-results/failing_results.json".  Retrying in 50.625 seconds...
2018-10-08 02:23:59,082 - Received HTTP status 400 loading "https://test-results.appspot.com/data/layout_results/mac10_13_retina-blink-rel/723/webkit_layout_tests (with patch)/layout-test-results/failing_results.json".  Retrying in 75.9375 seconds...
2018-10-08 02:25:15,048 - Received HTTP status 400 loading "https://test-results.appspot.com/data/layout_results/mac10_13_retina-blink-rel/723/webkit_layout_tests (with patch)/layout-test-results/failing_results.json".  Retrying in 113.90625 seconds...
2018-10-08 02:27:09,056 - Received HTTP status 400 loading "https://test-results.appspot.com/data/layout_results/mac10_13_retina-blink-rel/723/webkit_layout_tests (with patch)/layout-test-results/failing_results.json".  Retrying in 170.859375 seconds...
Traceback (most recent call last):
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/wpt_import.py", line 26, in <module>
    main()
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/wpt_import.py", line 19, in main
    host.exit(importer.main())
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/w3c/test_importer.py", line 173, in main
    if not self.update_expectations_for_cl():
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/w3c/test_importer.py", line 214, in update_expectations_for_cl
    self.fetch_new_expectations_and_baselines()
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/w3c/test_importer.py", line 569, in fetch_new_expectations_and_baselines
    self.rebaselined_tests, self.new_test_expectations = expectation_updater.update_expectations()
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/w3c/wpt_expectations_updater.py", line 86, in update_expectations
    port_results = self.get_failing_results_dict(build)
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/w3c/wpt_expectations_updater.py", line 137, in get_failing_results_dict
    layout_test_results = self.host.buildbot.fetch_results(build)
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/common/memoized.py", line 45, in __call__
    result = self._function(*args)
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/common/net/buildbot.py", line 113, in fetch_results
    full)
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/common/memoized.py", line 45, in __call__
    result = self._function(*args)
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/common/net/buildbot.py", line 159, in fetch_layout_test_results
    lambda: self.fetch_file('%s/%s' % (results_url, base_filename)))
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/common/net/network_transaction.py", line 61, in run
    self._check_for_timeout()
  File "/b/rr/tmpLuXNaI/w/src/third_party/blink/tools/blinkpy/common/net/network_transaction.py", line 68, in _check_for_timeout
    raise NetworkTimeout()
blinkpy.common.net.network_transaction.NetworkTimeout: NetworkTimeout

The first failing URL is "https://test-results.appspot.com/data/layout_results/mac10_13_retina-blink-rel/723/webkit_layout_tests (with patch)/layout-test-results/failing_results.json"

robertma@, since there is a space in that URL, possibly this has to do with the URL parsing changes you mentioned last week?