After upgrade to 12c, check if sqlnet.ora has below information
SQLNET.ALLOWED_LOGON_VERSION_SERVER = 11
SQLNET.ALLOWED_LOGON_VERSION_CLIENT = 11
SQLNET.ALLOWED_LOGON_VERSION_SERVER = 8
SQLNET.ALLOWED_LOGON_VERSION_CLIENT = 8
SQLNET.ALLOWED_LOGON_VERSION = 8
