Let's say the IBM MQ log contains something like this.

AMQ9665E: SSL connection closed by remote end of channel '????'.

This often means that the application attempting to connect to IBM MQ with SSL is using a certificate that failed the SSL handshake with the certificates in the IBM MQ Key Database file. Refer to IBM MQ - SSL for more information on IBM MQ and SSL.

Here is a more complete example of what you might see in the IBM MQ Error Log.

----- amqrccca.c : 439 --------------------------------------------------------
12/06/2022 01:16:20 AM - Process(3725.876159) User(mqm) Program(amqrmppa)
                    Host(server1.example.com) Installation(Installation1)
                    VRMF(9.1.0.10) QMgr(MANAGER01)
                    Time(2022-12-06T07:16:20.774Z)
                    RemoteHost(10.82.19.39)
                    ArithInsert1(420)
                    CommentInsert1(????)
                    CommentInsert2(gsk_secure_soc_init)
                    CommentInsert3(10.82.19.39 (10.82.19.39))
                   
AMQ9665E: SSL connection closed by remote end of channel '????'.

EXPLANATION:
The SSL or TLS connection was closed by the remote host 'ld20120ns
(10.82.19.39)' during the secure socket handshake. The channel is '????'; in
some cases its name cannot be determined and so is shown as '????'. The channel
did not start.
ACTION:
Check the remote end of the channel for SSL and TLS errors. Fix them and
restart the channel.
----- amqccisa.c : 7808 -------------------------------------------------------
12/06/2022 01:16:20 AM - Process(3725.876159) User(mqm) Program(amqrmppa)
                    Host(server1.example.com) Installation(Installation1)
                    VRMF(9.1.0.10) QMgr(MANAGER01)
                    Time(2022-12-06T07:16:20.774Z)
                    CommentInsert1(????)
                    CommentInsert2(3725)
                    CommentInsert3(10.82.19.39 (10.82.19.39))
                   
AMQ9999E: Channel '????' to host '10.82.19.39 (10.82.19.39)' ended abnormally.

EXPLANATION:
The channel program running under process ID 3725 for channel '????' ended
abnormally. The host name is '10.82.19.39 (10.82.19.39)'; in some cases the host
name cannot be determined and so is shown as '????'.
ACTION:
Look at previous error messages for the channel program in the error logs to
determine the cause of the failure. Note that this message can be excluded
completely or suppressed by tuning the "ExcludeMessage" or "SuppressMessage"
attributes under the "QMErrorLog" stanza in qm.ini. Further information can be
found in the System Administration Guide.
----- amqrmrsa.c : 945 --------------------------------------------------------