ORA-01102: cannot mount database in EXCLUSIVE mode

Problem

ORA-01102: cannot mount database in EXCLUSIVE mode during database instance startup

sqlplus / as sysdba
QL*Plus: Release 12.2.0.1.0 Production on Tue Apr 30 21:26:57 2019

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

Connected to an idle instance.

SQL> startup;
ORACLE instance started.

Total System Global Area 6174015488 bytes
Fixed Size                  8634320 bytes
Variable Size            1241514032 bytes
Database Buffers         4915724288 bytes
Redo Buffers                8142848 bytes
ORA-01102: cannot mount database in EXCLUSIVE mode

Solution

Same as https://sk.solutionmentors.com/2019/04/ora-01012-not-logged-on-startup-failed.html, seems this problem appeared because database was not stopped properly.

Follow great article http://www.dba-oracle.com/t_ora_01102_cannot_mount_database_in_exclusive_mode.htm

+++++++++++

POSSIBLE SOLUTION:
Verify that the database was shutdown cleanly by doing the following:

1. Verify that there is not a "sgadef<sid>.dbf" file in the directory
"ORACLE_HOME/dbs".

% ls $ORACLE_HOME/dbs/sgadef<sid>.dbf

If this file does exist, remove it.

% rm $ORACLE_HOME/dbs/sgadef<sid>.dbf

2. Verify that there are no background processes owned by "oracle"

% ps -ef | grep ora_ | grep $ORACLE_SID

If background processes exist, remove them by using the Unix
command "kill". For example:

% kill -9 <Process_ID_Number>

3. Verify that no shared memory segments and semaphores that are owned
by "oracle" still exist

% ipcs -b

If there are shared memory segments and semaphores owned by "oracle",
remove the shared memory segments

% ipcrm -m <Shared_Memory_ID_Number>

and remove the semaphores

% ipcrm -s <Semaphore_ID_Number>

NOTE: The example shown above assumes that you only have one
database on this machine. If you have more than one
database, you will need to shutdown all other databases
before proceeding with Step 4.

4. Verify that the "$ORACLE_HOME/dbs/lk<sid>" file does not exist. This is what caused issue in our case. Simple removal of this file did the trick.

5. Startup the instance

Related issues
https://sk.solutionmentors.com/2019/04/ora-01012-not-logged-on-startup-failed.html
https://sk.solutionmentors.com/2019/04/ora-27125-unable-to-create-shared.html

Reference
http://www.dba-oracle.com/t_ora_01102_cannot_mount_database_in_exclusive_mode.htm