Filed by: mlaza [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=373172458&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/VVk4jUd7QC6Jk8iW4D-p6Q/runs/0/artifacts/public/logs/live_backing.log

[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - TEST-PASS | /webdriver/tests/new_session/create_alwaysMatch.py | test_valid[strictFileInteractability-None] 
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - TEST-UNEXPECTED-FAIL | /webdriver/tests/new_session/create_alwaysMatch.py | test_valid[unhandledPromptBehavior-dismiss] - AssertionError: unknown error (500): Resource temporarily unavailable (os error 11)
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - new_session = <function fixture_new_session.<locals>.new_session at 0x7ff2d8fc99d8>
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - add_browser_capabilities = <function fixture_add_browser_capabilities.<locals>.add_browser_capabilities at 0x7ff2d877ae18>
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - key = 'unhandledPromptBehavior', value = 'dismiss'
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - 
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -     @pytest.mark.parametrize("key,value", flatten(product(*item) for item in valid_data))
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -     def test_valid(new_session, add_browser_capabilities, key, value):
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -         response, _ = new_session({"capabilities": {
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -             "alwaysMatch": add_browser_capabilities({key: value})}})
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - >       assert_success(response)
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - 
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - _          = None
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - add_browser_capabilities = <function fixture_add_browser_capabilities.<locals>.add_browser_capabilities at 0x7ff2d877ae18>
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - key        = 'unhandledPromptBehavior'
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - new_session = <function fixture_new_session.<locals>.new_session at 0x7ff2d8fc99d8>
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - response   = <Response status=500 error=<UnknownErrorException http_status=500>>
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - value      = 'dismiss'
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - 
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - tests/web-platform/tests/webdriver/tests/new_session/create_alwaysMatch.py:15: 
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - 
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - response = <Response status=500 error=<UnknownErrorException http_status=500>>
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - value = None
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO - 
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -     def assert_success(response, value=None):
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -         """
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -         Verify that the provided webdriver.Response instance described
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -         a valid success response as defined by `dfn-send-a-response` and
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -         the provided response value.
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -     
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -         :param response: ``webdriver.Response`` instance.
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -         :param value: Expected value of the response body, if any.
[task 2022-04-02T07:34:56.643Z] 07:34:56     INFO -         """
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - >       assert response.status == 200, str(response.error)
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - E       AssertionError: unknown error (500): Resource temporarily unavailable (os error 11)
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - E         
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - E       assert 500 == 200
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - E         +500
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - E         -200
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - 
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - response   = <Response status=500 error=<UnknownErrorException http_status=500>>
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - value      = None
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - 
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - tests/web-platform/tests/webdriver/tests/support/asserts.py:69: AssertionError
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - ........
[task 2022-04-02T07:34:56.644Z] 07:34:56     INFO - TEST-OK | /webdriver/tests/new_session/create_alwaysMatch.py | took 154500ms

Might be a side-effect from a failure when dismissing a prompt. Lets wait for the fix on bug 1762543.