Oracle 12.2: Reset user password is not working

Question

I have tried to import a database-dump that was created with Oracle 12.1 (12c Release 1) into my Oracle 12.2.

There was some problem due to a false or invalid authentication protocol:

ORA-28040: No matching authentication protocol

I found some posts where people had similar problems and tried to fix it by adding the following two lines to my sqlnet.ora (the file is located in C:\app\oracle\product\12.2.0\dbhome_1\network\admin):

SQLNET.ALLOWED_LOGON_VERSION = 8
SQLNET.ALLOWED_LOGON_VERSION_SERVER = 8

Since then I can not connect with any of my users anymore. I have deleted the two lines above from sqlnet.ora and now my sqlnet.ora only contains the following two lines, just as before:

SQLNET.AUTHENTICATION_SERVICES = (NTS)
NAMES.DIRECTORY_PATH= (TNSNAMES, ONAMES, HOSTNAME)

But I cannot connect with any user now. I thought I just have to reset the passwords for all of my users.

So I have tried to reset the passwords for all Oracle users. Therefore I use the cmd and log in to sqlplus as sysdba:

C:\>sqlplus / as sysdba
SQL*Plus: Release 12.2.0.1.0 Production on Mi Nov 20 14:14:44 2019

Copyright (c) 1982, 2016, Oracle.  All rights reserved.


Connected to:
Oracle Database 12c Standard Edition Release 12.2.0.1.0 - 64bit Production

SQL> select * from all_users where username='CUSTOMER';

USERNAME USER_ID CREATED  COM O INH DEFAULT_COLLATION IMP ALL
--------------------------------------------------------------
CUSTOMER 106 10.07.19 NO  N NO USING_NLS_COMP NO  NO

SQL> alter user CUSTOMER identified by CUST;

User was changed.

SQL> connect CUSTOMER/CUST
ERROR:
ORA-01017 Invalid Username/Password

The same happens if I try to reset the password in the GUI of SQL Developer. I always get ORA-01017.

With another user it worked again, but only after I tried the following:

alter user CUST2 identified by "cust2";

So I had to put the password in double quotes and in the properties of the connection in SQL Developer I used the password cust2 without quotes for the user CUST2. Somehow that worked, but I have tried the following options for the password for the user CUSTOMER: CUST, "CUST", cust, "cust". I have also tried different passwords like TEST, test, "Test", custom, "custom", etc.

But nothing works, I can not connect with the user CUSTOMER anymore.

The SID of my Oracle instance is orcl, and I have also tried the following:

SQL> connect CUSTOMER@orcl/CUST
ERROR:
ORA-12514 – 
TNS:listener does not currently know of service requested in connect descriptor

Does someone know how to fix this?