Solution 1: The Oracle server is down, please ensure that the server is up and running.

Solution 2: The Oracle server is not accessible from the Applications Manager machine. Verify the given host and port by executing the following command from the machine in which Applications Manager server is running:

telnet oracle_hostname oracle_portnumber

Refer Oracle Support KB

Solution 1: The Instance Name (Service Name) specified in Applications Manager is wrong. Try to connect to Oracle using SQLPlus and "Connect String/Service Name" should be given as Instance Name in Applications Manager when you are connecting through the SQLPlus.

1. The instance name can also be found out from the <oracle-installation directory\network\admin\listener.ora>. In <listener.ora> file, you will have an entry <SID_NAME = orcl>, where orcl is the Instance name.
2. The instance name can also be found out from the<oracle-installation directory\network\admin\tnsnames.ora>. In <tnsnames.ora> file, you will have an entry <SERVICE_NAME = orcl>, where orcl is the Instance name.

Solution 2: Check if the Oracle instance is running. Check if the Oracle server is running and accessible from the Applications Manager server.

Solution 3: Check if the Oracle listener is running. If the database is not registered with the listener, register it manually to allow connection.

Solution: A user with CONNECT and SELECT_CATALOG_ROLE roles is required for monitoring by Applications Manager. So configure a user with these permissions and try with that user in Applications Manager.

Solution: In the Oracle DB server installation directory, edit the sqlnet.ora file (present under <oracle home>\network\admin directory) and remove the entry SQLNET.ALLOWED_LOGON_VERSION=10 in that file or set to lower version. Then restart Oracle DB server and try to add it in Applications Manager.

Solution 1: Make sure you are connecting to the correct port, a wrong port can also give the error mentioned above. Also, check if the SID used to connect in tnsnames.ora and Listener.ora are correct.

Solution 2: It seems you have enabled ACL on the listener. You can follow these steps to avoid the error:

a. Modify $ORACLE_HOME/network/admin/sqlnet.ora file with following parameter: 

tcp.validnode_checking = no

b. If you don't want to disable this, you can put the machine names (provide Applications Manager hostname) as follows and restart the listener:

tcp.invited_nodes=(machine1, machine2)

Solution 3: Check the number of processes and shared pool size.

Show parameter SHARED_POOL_SIZE;

Show parameter Processes;

If the values are insufficient, increase the size using:

Alter system set SHARED_POOL_SIZE=<size the environment demands>M;

Alter system set PROCESSES=<Size the environment demands> SCOPE=SPFILE;

Solution 4: If you have enabled Oracle SSL option but missed to give the same while adding the monitor then this error will occur.

Note: If none of the above solutions fix the problem, execute the file OracleDebug.bat under the <AppManager_Home>\bin\troubleshooting directory and send the result.

          Solution: A trigger is configured to fire on failed login attempts. When an incorrect login attempt occurs, the trigger execution is initiated. However, the user whose login attempt failed does not have the necessary permissions to execute the trigger, causing an error. The actual issue is an incorrect username/password.

1. Error Message 3: -br-ORA-17503:ksfdopn:2 Failed to open  file+FILELOCATION. ORA-15173: entry ' ' does not exist in directory   

• Check if the password file exists and has proper permissions. If missing, recreate it.

• Run the following query to check if the database is using password file authentication:

• SHOW PARAMETER REMOTE_LOGIN_PASSWORDFILE;
  

1. If it returns NONE, oracle does not use a password file, and you must use OS authentication (/ as sysdba).
   
2. If it returns EXCLUSIVE or SHARED, then Oracle expects a valid password file.
   

Error : ORA-00257: archiver error. Connect internal only, until freed.

Cause: The archive log destination is full, preventing Oracle from writing archived redo logs. Normal users cannot connect until space is freed.

Solution: 

1. Free space by deleting old archive logs or increasing db_recovery_file_dest_size. SYSDBA can connect to resolve the issue.

2. Ref Oracle support KB
   
   

Cause: A recursive SQL error occurred due to a trigger, security policy, or system constraint. In this case, ORA-20605 indicates that the user does not have permission to access the database, likely enforced by a logon trigger.

Solution:

1. Ensure the user has the necessary database access permissions.

2. Also, follow the steps provided in the KB
   
   

Cause: Oracle requires a username for login, but none was provided. This happens when an application tries to connect without specifying a username.

Solution: Ensure the username is provided explicitly when connecting to the database.

Cause: This error occurs when the Oracle client tries to establish a TCPS (SSL/TLS) connection, but the server does not support or recognize the SSL handshake. It can be due to missing TCPS listener configuration, incorrect connection protocol, or SSL misconfiguration. 

Solution: 

1. Ensure the listener is configured for TCPS (lsnrctl status), and check sqlnet.ora and listener.ora for correct SSL settings.

2. Ref KB to add SSL enabled Oracle.
   

Cause:  This error occurs when trying to connect to a down database as a non SYSDBA USER. The Oracle database instance is not started, leading to connection failure.

Solution:

1. Start the Oracle database

2. Ensure ORACLE_SID and ORACLE_HOME are correctly set and check shared memory availability

3. Ref Oracle support KB.
   
   
   

Solution:

             Check network connectivity, database availability, and listener status to ensure the connection completes within the timeout.

Share the below details along with the screenshots/reports mentioned on each step based on the error message user receives:

If the issue persists after following the above steps, collect and share the following details:

1. Issue reported Oracle monitor → Edit monitor/Add monitor →Test credential result screenshot.

2. Health and Availability history report

3. OracleDebug script result

4. Oracle version, SSL enabled

5. Connect the Oracle DB Server and execute the below command and share the result

1. lsnrctl status