The recent plugin manager work broke the base drupal install and the main admin/ menu. A few people noticed while Drupal.org and update.drupal.org were sending 503 errors that we had XML errors while installing and the admin/ menu.
The bug is kind of hard to reproduce, because you need update.drupal.org to return something that is not valid XML while still returning something.
The solution is to not flush the caches so aggressively, but only if we can actually fetch the updates from update.drupal.org. We double-check we receive a proper status code from the server before treating its data as proper.
PASSED: [[SimpleTest]]: [MySQL] 31,572 pass(es).
FAILED: [[SimpleTest]]: [MySQL] 31,565 pass(es), 2 fail(s), and 6 exception(es).
FAILED: [[SimpleTest]]: [MySQL] Drupal installation failed.
FAILED: [[SimpleTest]]: [MySQL] Unable to apply patch common.inc__23.patch.
Failed: Failed to apply patch.