what are the PASSWORD_VERSIONS for these accounts ? you can check that from dba_users view . it seems these accounts are using old hashing algorithms. From 12cR2 and beyond oracle enforced higher authentication level !
in my opinion you have two options:
- reset the password of these accounts (password_versions) value should be updated, and make sure your application client is compatible with 18c !
- lower the authentication by setting the following parameter in sqlnet.ora file for example : SQLNET.ALLOWED_LOGON_VERSION=11
first option (resetting the password) worked with me fine ! Thank You !