Consistent failures for a specific patch

I keep having the exact same failure on the testbot https://qa.drupal.org/pifr/test/989138

The same thing has happened on this issue with the last few patches https://www.drupal.org/node/2427335 which are all the same, just re-uploaded after a rebase.

A point in the right direction would be good, the test that fails, the error message in the log, etc.