Invisible UNIQUE index

[Martin Lechner]

(1.) Part of dump without data - One Table

DROP TABLE IF EXISTS `terminalconfiguration_crypto_configuration`;
/*!40101 SET @saved_cs_client = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `terminalconfiguration_crypto_configuration` (
`tconfiguration_id` bigint(20) NOT NULL,
`cc_id` bigint(20) NOT NULL,
KEY `FKF428C11D7349287E` (`tconfiguration_id`),
KEY `FKF428C11DBDE83D1F` (`cc_id`),
CONSTRAINT `FK70pc36pkrt9x7lwvdf8796bcb` FOREIGN KEY (`tconfiguration_id`) REFERENCES `terminalconfiguration` (`id`),
CONSTRAINT `FKF428C11D7349287E` FOREIGN KEY (`tconfiguration_id`) REFERENCES `terminalconfiguration` (`id`),
CONSTRAINT `FKF428C11DBDE83D1F` FOREIGN KEY (`cc_id`) REFERENCES `tcryptoconfiguration` (`id`),
CONSTRAINT `FKm8d00sxnh9g09wqb05goi1dk2` FOREIGN KEY (`cc_id`) REFERENCES `tcryptoconfiguration` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */;


(2.) select * from information_schema.TABLE_CONSTRAINTS where table_name = 'terminalconfiguration_crypto_configuration' and constraint_type = 'UNIQUE';

(3.) Result of select (2):
CONSTRAINT CATALOG: def
CONSTRAINT_SCHEMA: batm
CONSTRAINT_NAME: UK_oux3v44rv2m0rtmgkrsknlv10
TABLE_SCHEMA: batm
TABLE_NAME: terminalconfiguration_crypto_configuration
CONSTRAINT_TYPE: UNIQUE

(4.) DROP INDEX `UK_oux3v44rv2m0rtmgkrsknlv10` ON `terminalconfiguration_crypto_configuration`;

(5.) Error of command (4):
Error: 1025, SQLState: HY000
Error on rename of './batm/#sql-4e5_4c' to './batm/terminalconfiguration_crypto_configuration' (errno: 150 - Foreign key constraint is incorrectly formed)

Encoding and engine are same in all tables (InnoDb, utf8). All PK IDs are bigint(20). It functions on many databases, but one our customer has this error. We cannot delete unique index and also it is not exported schema. How can I delete it? (Why is the index not in the exported schema?)