ORA-02391: exceeded simultaneous SESSIONS_PER_USER limit

$ sqlplus / as sysdba

SQL*Plus: Release 12.1.0.2.0 Production on Thu Nov 21 19:20:04 2019

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Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

Check the profile of the user experiencing the issue.

SQL>
SQL> select username , profile from dba_users where username = 'MUF8127ADAP';

USERNAME
--------------------------------------------------------------------------------
PROFILE
--------------------------------------------------------------------------------
MUF8127ADAP

DEFAULT

Check the Limit of the profile.

SQL> set pages 200 lines 700
SQL> col LIMIT for a30
SQL> col RESOURCE_NAME for a30
SQL> ;
  1* select resource_name, limit from dba_profiles where profile='DEFAULT'
SQL> select resource_name, limit from dba_profiles where profile='DEFAULT';

RESOURCE_NAME                  LIMIT
------------------------------ ------------------------------
COMPOSITE_LIMIT                UNLIMITED
SESSIONS_PER_USER              10
CPU_PER_SESSION                UNLIMITED
CPU_PER_CALL                   UNLIMITED
LOGICAL_READS_PER_SESSION      UNLIMITED
LOGICAL_READS_PER_CALL         UNLIMITED
IDLE_TIME                      UNLIMITED
CONNECT_TIME                   UNLIMITED
PRIVATE_SGA                    UNLIMITED
FAILED_LOGIN_ATTEMPTS          5
PASSWORD_LIFE_TIME             90
PASSWORD_REUSE_TIME            365
PASSWORD_REUSE_MAX             12
PASSWORD_VERIFY_FUNCTION       NULL
PASSWORD_LOCK_TIME             1
PASSWORD_GRACE_TIME            14

16 rows selected.

Increase the limit accordingly. Here it is being set to unlimited.

SQL> alter profile default limit SESSIONS_PER_USER unlimited;

Profile altered.

SQL> select resource_name, limit from dba_profiles where profile='DEFAULT'
  2  ;

RESOURCE_NAME                  LIMIT
------------------------------ ------------------------------
COMPOSITE_LIMIT                UNLIMITED
SESSIONS_PER_USER              UNLIMITED
CPU_PER_SESSION                UNLIMITED
CPU_PER_CALL                   UNLIMITED
LOGICAL_READS_PER_SESSION      UNLIMITED
LOGICAL_READS_PER_CALL         UNLIMITED
IDLE_TIME                      UNLIMITED
CONNECT_TIME                   UNLIMITED
PRIVATE_SGA                    UNLIMITED
FAILED_LOGIN_ATTEMPTS          5
PASSWORD_LIFE_TIME             90
PASSWORD_REUSE_TIME            365
PASSWORD_REUSE_MAX             12
PASSWORD_VERIFY_FUNCTION       NULL
PASSWORD_LOCK_TIME             1
PASSWORD_GRACE_TIME            14

16 rows selected.