"Credential retrieval failed" during recovery of an Oracle database
I've just solved the same error on Oracle12c under Windows on my test enviroment.
The Oracle service was running under oracle user local account but I was trying the PITR with my domain user. Even if my domain user was member of ORADBA group I've got the same nasty ORA-12638 error.

I've changed the user account of the Oracle Service and got it finally working.
I guess if you use SYSTEM account it will work as well.

Regards,
Daniel.