I tried to update "Herman Brood's 50: The Soundtrack" to use the new credits. That took some time (adding new artists &c.), the debug output:

Internal Server Error

Oops, something went wrong!

Error:

Caught exception in MusicBrainz::Server::Controller::ReleaseEditor::Edit->edit "Can't use an undefined value as a HASH reference at lib/MusicBrainz/Server/Wizard.pm line 269."

Stack trace:

line 269 MusicBrainz::Server::Wizard
line 773 MusicBrainz::Server::Wizard::ReleaseEditor
line 76 MusicBrainz::Server::Wizard::ReleaseEditor
line 169 MusicBrainz::Server::Wizard
line 402 MusicBrainz::Server::Wizard
line 133 MusicBrainz::Server::Wizard
line 93 MusicBrainz::Server::Wizard::ReleaseEditor
line 17 MusicBrainz::Server::Controller::ReleaseEditor::Edit
line 307 MusicBrainz::Server
line 307 MusicBrainz::Server
line 307 MusicBrainz::Server
line 271 MusicBrainz::Server

Request data:

$VAR1 = {
'query_parameters' => {},
'body_parameters' => {
'rec_mediums.0.associations.3.confirmed' => '1',
'rec_mediums.0.associations.10.confirmed' => '1',
'rec_mediums.0.associations.2.gid' => 'new',
'rec_mediums.0.associations.5.gid' => 'b10aee56-90f1-4efe-8a09-eab1fe38af36',
'rec_mediums.0.tracklist_id' => '123972',
'rec_mediums.0.associations.11.gid' => 'c34afba6-2306-4566-ac13-4506105a98d6',
'rec_mediums.0.associations.6.confirmed' => '1',
'rec_mediums.0.associations.0.edit_sha1' => 'tdH7O7sm7FX5CvuinAwW3fDf8P0',
'propagate_all_track_changes' => '1',
'rec_mediums.0.associations.14.update_recording' => '1',
'select-recording-0-5' => 'on',
'rec_mediums.0.associations.11.update_recording' => '1',
'rec_mediums.0.associations.0.confirmed' => '1',
'rec_mediums.0.associations.0.update_recording' => '1',
'rec_mediums.0.associations.9.gid' => 'bff8faba-4247-4ec7-9cb1-d7a201e210d4',
'rec_mediums.0.associations.14.confirmed' => '1',
'rec_mediums.0.associations.7.gid' => '3d5d06f7-9c89-489e-a489-db62020ca381',
'rec_mediums.0.associations.6.update_recording' => '1',
'rec_mediums.0.associations.4.update_recording' => '1',
'rec_mediums.0.associations.1.edit_sha1' => '9vnIebhJKWrR8N/Ci5lptMGkHz4',
'select-recording-0-3' => 'on',
'select-recording-0-0' => 'on',
'select-recording-0-15' => 'on',
'select-recording-0-12' => 'on',
'rec_mediums.0.associations.8.confirmed' => '1',
'rec_mediums.0.associations.11.confirmed' => '1',
'rec_mediums.0.associations.12.edit_sha1' => 'tGcNoUMY/kwjNzZ5E6+Pk/eOFeM',
'rec_mediums.0.associations.6.edit_sha1' => 'RYCQvWB2WZiiikZ51lLdS1Y5zCs',
'rec_mediums.0.associations.12.confirmed' => '1',
'rec_mediums.0.associations.4.confirmed' => '1',
'rec_mediums.0.associations.8.edit_sha1' => '4yn5tEookwzh7+aqM3qBRRVyOS8',
'rec_mediums.0.associations.7.update_recording' => '1',
'rec_mediums.0.associations.1.update_recording' => '1',
'rec_mediums.0.associations.15.confirmed' => '1',
'rec_mediums.0.associations.13.edit_sha1' => 'zANO/GSjLPAX8zsXujSl3J6wY5o',
'wizard_session_id' => '0.552292175448628',
'rec_mediums.0.associations.8.update_recording' => '1',
'rec_mediums.0.associations.2.confirmed' => '1',
'rec_mediums.0.associations.4.edit_sha1' => '1c8KxVpJcQbCfiAV+JwhUJnVxBk',
'select-recording-0-11' => 'on',
'rec_mediums.0.associations.5.update_recording' => '1',
'rec_mediums.0.associations.3.edit_sha1' => 'BaauI9mV6W4betew5fNXYZ6GoHc',
'next' => "Next \x

",
'rec_mediums.0.associations.13.update_recording' => '1',
'select-recording-0-2' => 'on',
'select-recording-0-8' => 'on',
'rec_mediums.0.associations.14.edit_sha1' => 'BpjwPgILr+MEA3t5HcVddjblV0o',
'select-recording-0-1' => 'on',
'rec_mediums.0.associations.11.edit_sha1' => '9dL2a8jNh4H4ALO0nUZCANig2I4',
'rec_mediums.0.associations.3.gid' => '50379ef9-6221-456b-a596-e61109a37680',
'rec_mediums.0.associations.1.confirmed' => '1',
'rec_mediums.0.associations.13.confirmed' => '1',
'rec_mediums.0.associations.14.gid' => 'f2b9bd35-2211-47d7-ab48-a8915e3bda1d',
'rec_mediums.0.associations.7.edit_sha1' => 'HDEQak5F8wM70fUAbaJww4Y8uo0',
'rec_mediums.0.associations.1.gid' => '8ae722d6-1b1c-4927-8f23-1f9edbe5df7c',
'rec_mediums.0.associations.12.gid' => '71ff2e72-6ba9-4ae4-9844-7713f78b8d4f',
'rec_mediums.0.associations.12.update_recording' => '1',
'rec_mediums.0.associations.15.update_recording' => '1',
'rec_mediums.0.associations.7.confirmed' => '1',
'rec_mediums.0.associations.5.edit_sha1' => '1kd5/pbIgbKUSlsTYG6NkHxZZec',
'select-recording-0-13' => 'on',
'select-recording-0-6' => 'on',
'rec_mediums.0.associations.8.gid' => '6fdf14ff-80d8-4698-b4d7-9a3bf9c9217a',
'select-recording-0-4' => 'on',
'rec_mediums.0.associations.6.gid' => 'df343ca7-f859-40ef-a062-0ec5d8b751db',
'rec_mediums.0.associations.15.gid' => 'f2f89534-0824-4106-a806-59a3f08e6769',
'rec_mediums.0.associations.10.gid' => '665831c7-4432-463c-97db-5e2a87390d4d',
'rec_mediums.0.associations.5.confirmed' => '1',
'rec_mediums.0.associations.4.gid' => '99bf4575-0e89-4b3e-bcb3-8e750497fac8',
'select-recording-0-10' => 'on',
'select-recording-0-7' => 'on',
'select-recording-0-9' => 'on',
'wizard_page_id' => '2',
'rec_mediums.0.associations.15.edit_sha1' => '7k8GrMhRJ6Q0cksZIfWd753lCx4',
'rec_mediums.0.associations.13.gid' => '7d86734f-a4af-4bc1-96b4-1ab99c81d667',
'rec_mediums.0.associations.2.edit_sha1' => 'bYfqUBs1IvYFwkldKr8LEOqHbCQ',
'rec_mediums.0.associations.2.update_recording' => '1',
'rec_mediums.0.associations.3.update_recording' => '1',
'rec_mediums.0.associations.9.confirmed' => '1',
'rec_mediums.0.associations.0.gid' => '060b22c2-3ba9-4f18-8e3a-d7286e55b08e',
'rec_mediums.0.associations.9.edit_sha1' => 'TubPibSrrjmEMq5usmb4uqM0Yxo',
'rec_mediums.0.associations.10.edit_sha1' => 'e7VmYuMTiO1Vtf4Px2oCZRQP58s',
'select-recording-0-14' => 'on'
}
};

URL: http://musicbrainz.org/release/570c3673-4382-49b5-a81f-00fc9fe66502/edit