Intermittent 8.0.x HEAD and patch failures on #3083 (bot disabled)

Bot #3083 appears to be failing patches and HEAD intermittently with:

Overall Summary
==============================================

FAILED: [[SimpleTest]]: [PHP 5.4 MySQL] 94,358 pass(es), 1,470 fail(s), and 665 exception(s).

Individual Environment Summaries
==============================================

-- [[SimpleTest]]: [PHP 5.4 MySQL] --

* Drupal\aggregator\Tests\FeedLanguageTest (18 pass(es), 13 fail(s), and 7 exception(s))
   - [fail] [Completion check] "The test did not complete due to a fatal error." in FeedLanguageTest.php on line 54 of Drupal\aggregator\Tests\FeedLanguageTest->testFeedLanguage().
   - [fail] [Browser] "GET http://ec2-52-26-207-233.us-west-2.compute.amazonaws.com/checkout/user/logout?destination=user/login returned 0 (0 bytes)." in FeedLanguageTest.php on line 56 of Drupal\aggregator\Tests\FeedLanguageTest->testFeedLanguage().
   - [fail] [Browser] "User was logged out." in FeedLanguageTest.php on line 56 of Drupal\aggregator\Tests\FeedLanguageTest->testFeedLanguage().
   - [exception] [Warning] "simplexml_import_dom(): Invalid Nodetype to importsimplexml_import_dom(Object)

[...etc]

@alexpott disabled the bot:
3038 Disabled Test http://ec2-52-7-248-91.compute-1.amazonaws.com/ view test | request enable | edit | remove

We did not determine what the specific issue was. Leaving at major for now; if other bots are also affected this should be promoted to critical and investigated further.