The mqs.ini file, which is typically located at /var/mqm/mqs.ini, contains the base logs directory. By default, mqs.ini has the following:

LogDefaults:
   LogDefaultPath=/var/mqm/log

By default, the general error logs are located below /var/mqm/errors and have the following name format.

/var/mqm/errors/AMQERR01.LOG
/var/mqm/errors/AMQERR02.LOG
/var/mqm/errors/AMQERR03.LOG

By default, the queue manager error logs are located below /var/mqm/qmgrs/your_manager/errors and have the following name format.

/var/mqm/qmgrs/MANAGER01/errors/AMQERR01.LOG
/var/mqm/qmgrs/MANAGER01/errors/AMQERR02.LOG
/var/mqm/qmgrs/MANAGER01/errors/AMQERR03.LOG

Here is an example of one possible exception in the Error Log, which identifies that channel "MANAGER01.CHANNEL01.SVR" was blocked. In this scenario, you would use the display chlauth command to confirm that "client.example.com" does indeed not have access to channel "MANAGER01.CHANNEL01.SVR".

mm/dd/yyyy hh:mm:ss - Process(18271.9654) User(mqm) Program(amqrmppa)
                    Host(mq.example.com) Installation(Installation1)
                    VRMF(9.1.0.3) QMgr(MANAGER01)
                    Time(yyyy-mm-ddThh:mm:ss.sss)
                    RemoteHost(10.1.2.3)
                    CommentInsert1(MANAGER01.CHANNEL01.SVR)
                    CommentInsert2(SERVER01 (10.4.5.6))
                    CommentInsert3(CLNTUSER(john.doe) ADDRESS(client.example.com))

AMQ9777E: Channel was blocked

EXPLANATION:
The inbound channel 'MANAGER01.CHANNEL01.SVR' was blocked from address 'client.example.com
(10.4.5.6)' because the active values of the channel matched a record
configured with USERSRC(NOACCESS). The active values of the channel were
'CLNTUSER(john.doe) ADDRESS(client.example.com)'.