drush cronが失敗する：回復不可能なエラーが原因で、Drushコマンドが異常終了しました
$ drush -v -d cron
Bootstrap to phase 0. [0 sec, 1.65 MB] [bootstrap]
Drush bootstrap phase : _drush_bootstrap_drush() [0 sec, 1.77 MB] [bootstrap]
Cache HIT cid: 5.4-commandfiles-0-33f01882e55343a61f1cb10880b0da0f [0.01 sec, 1.78 MB] [debug]
Bootstrap to phase 0. [0.02 sec, 4.16 MB] [bootstrap]
Bootstrap to phase 6. [0.02 sec, 4.16 MB] [bootstrap]
Drush bootstrap phase : _drush_bootstrap_drupal_root() [0.02 sec, 4.17 MB] [bootstrap]
Initialized Drupal 7.28 root directory at ...htdocs [0.03 sec, 5.78 MB] [notice]
Drush bootstrap phase : _drush_bootstrap_drupal_site() [0.03 sec, 5.78 MB] [bootstrap]
Initialized Drupal site default at sites/default [0.03 sec, 5.79 MB] [notice]
Cache HIT cid: 5.4-commandfiles-2-c32acd775c18a90c5b41d0fc9accf5ab [0.03 sec, 5.79 MB] [debug]
Drush bootstrap phase : _drush_bootstrap_drupal_configuration() [0.03 sec, 5.79 MB] [bootstrap]
Drush bootstrap phase : _drush_bootstrap_drupal_database() [0.03 sec, 5.8 MB] [bootstrap]
Successfully connected to the Drupal database. [0.03 sec, 5.8 MB] [bootstrap]
Drush bootstrap phase : _drush_bootstrap_drupal_full() [0.04 sec, 6.3 MB] [bootstrap]
Cache HIT cid: 5.4-commandfiles-5-21916586faf556c236c401ff89ea50f6 [0.18 sec, 33.52 MB] [debug]
Drush bootstrap phase : _drush_bootstrap_drupal_login() [0.18 sec, 34.19 MB] [bootstrap]
Successfully logged into Drupal as Anonymous (uid=0) [0.18 sec, 34.2 MB] [bootstrap]
Found command: core-cron (commandfile=core) [0.18 sec, 34.2 MB] [bootstrap]
Drush command terminated abnormally due to an unrecoverable error. [0.2 sec, 34.87 MB] [error]
admin/reports/dblogには何も表示されません。
/cron.php?cron_key=...への移動は、Webインターフェイスから正常に機能することに注意してください。しかし、コンソールからカールすると失敗します。 （私は https://github.com/drush-ops/drush/issues/664 でも問題を報告しましたが、ここで指示されました。）
何が問題なのかを知るにはどうすればよいですか？