These appear to be the corresponding log from the api server production.log (there does not appear to be any mention of the issue in our postgres logs):
[api.arvados.sanger] [42516ad406ffb11b68c6b7c720679436] WARNING: Can't verify CSRF token authenticity
[api.arvados.sanger] [42516ad406ffb11b68c6b7c720679436] #<ActiveRecord::StatementInvalid: PG::InternalError: ERROR: invalid memory alloc request size 1718630765
[api.arvados.sanger] [42516ad406ffb11b68c6b7c720679436] /var/www/arvados-api/shared/vendor_bundle/ruby/2.1.0/gems/activerecord-3.2.22.5/lib/active_record/connection_adapters/postgresql_adapter.rb:1176:in `get_last_result'
[api.arvados.sanger] [42516ad406ffb11b68c6b7c720679436] Error 1489149789+85807e44: 422
[api.arvados.sanger] [42516ad406ffb11b68c6b7c720679436] {"method":"POST","path":"/arvados/v1/job_tasks","format":"json","controller":"arvados/v1/job_tasks","action":"create","status":422,"duration":32.56,"view":0.45,"db":18.46,"params":{"parameters":{"inputs":"5c41dcf66d94aba8b7c1553c39a4520c+23030","name":"55_of_200","interval":"chr5:91121972-109108305","interval_list":"a65feae6f5f8dc407f422586aed7dc26+96","ref":"9a15a1d495a6efa0f3e05f1e851b694e+2227","reuse_job_task":"z8ta6-ot0gb-vi32h3s1a61u01e"},"success":true,"sequence":2,"finished_at":"2017-03-09T21:32:59.000000000Z","created_by_job_task_uuid":"z8ta6-ot0gb-bovzdgts2cmm8i2","progress":1.0,"output":"d2f2c39e81e617b139da9f6ea5cf581a+3511+A8e3cc4d710260d397eea250fdb048f58f0a53c8b@58d43f08","started_at":"2017-03-09T19:07:16.000000000Z","job_uuid":"z8ta6-8i9sb-1u981ftt6rzlgbb","alt":"json","job_task":{"job_uuid":"z8ta6-8i9sb-1u981ftt6rzlgbb","sequence":2,"parameters":{"inputs":"5c41dcf66d94aba8b7c1553c39a4520c+23030","name":"55_of_200","interval":"chr5:91121972-109108305","interval_list":"a65feae6f5f8dc407f422586aed7dc26+96","ref":"9a15a1d495a6efa0f3e05f1e851b694e+2227","reuse_job_task":"z8ta6-ot0gb-vi32h3s1a61u01e"},"output":"d2f2c39e81e617b139da9f6ea5cf581a+3511+A8e3cc4d710260d397eea250fdb048f58f0a53c8b@58d43f08","progress":1.0,"success":true,"created_by_job_task_uuid":"z8ta6-ot0gb-bovzdgts2cmm8i2","started_at":"2017-03-09T19:07:16.000000000Z","finished_at":"2017-03-09T21:32:59.000000000Z"}},"@timestamp":"2017-03-10T12:43:09Z","@version":"1","message":"[422] POST /arvados/v1/job_tasks (arvados/v1/job_tasks#create)"}