What reasons are there for a transfer failed status during an incoming transfer?

If a transfer fails with the error message “TRANSFER FAILED” you will get additional information which the have following meaning:

Domain is locked
Domain status does not allow for operation; Domain has status TransferProhibited
To transfer the domain to inwx, the transfer lock at the previous provider must be removed. Please consult the current administrator.

Invalid authorization information
To transfer the domain us, it is necessary to have an AUTHCode (Authentification-Code). The code you have entered was invalid. Please consult the current administrator.

Transfer was not authorized by owner/admin contact. Please check whois email address.
The link that has been sent to the owner and/or admin contact, was not clicked at and therefor not verified. Further information can be retrieved here.