The log shows three different and unrelated issues.
AMQ9532: The program cannot set the queue attributes.
Explanation: An attempt to set the attributes of the queue 'SYSTEM.CLUSTER.TRANSMIT.QUEUE' to the queue manager "QMGR.SERVER6A" failed with reason code 2102.
2102 is MQRC_RESOURCE_PROBLEM and, presumably, the resource problem mentioned in the wiring. 2102 can be any scarce resource, including semaphores, user processes, queue handles, etc. Since QMgr was trying to set the queue attribute, it would already have a thread, but it would require additional queue handles. When something happens, use your admin tool (WMQ Explorer, mqmon or one of many third-party tools) to view the number of handles open line, open channels, etc. Please note that for a resource error this will be necessary to maintain an open connection with QMgr, otherwise the tool will not be able to create a new connection when there is a lack of resources.
AMQ9999: The channel program terminated abnormally.
EXPLANATION: Channel.Coord00 program ended abnormally.
ACTION: Look at the previous error message for the channel program 'C00.US.MP00' in the error files to determine the cause of the failure.
This error is actually two different errors, because it refers to two different channels. One of them appears to be an outbound channel of the cluster, and the other appears to be a point-to-point channel. None of the channels mentioned in this error are associated with the first and last error message.
06/06/11 08:24:26 AMQ9544: Messages are not placed on the destination queue.
EXPLANATION: During processing of the channel "Channel.Server6A" one or more messages could not be the target queue and attempts to put them in the dead letter queue. The queue location is 1, where 1 is the local dead letter queue and 2 is the deleted dead letter queue.
ACTION: Examine the contents of the dead letter queue. Each message is contained in a structure that describes why the message was queued, and where it was originally addressed. Also look at previous error messages to see if an attempt to send messages to a dead letter queue has failed. Program identifier (PID) processing program was "1372200".
The last error is the inbound channel of the cluster. Since the first error attempted to set the attributes of the cluster transfer queue, it could only be associated with the outbound channel. Therefore, the first and last error message are not related. This error message will appear to display an incoming message destined for the queue, and this queue has been full, the PUT has been disabled, or otherwise it cannot receive the message. Therefore, the message was sent to the dead letter queue.
For a resource error, I would suggest looking at the performance report that matches your platform. Go to the SupportPacs page and find those SupportPacs named MP *, and then search for them for your platform. Performance reports give you specific tuning advice.
You can also read the Problem Definition chapter in the System Administration guide for more recommendations on identifying resource problems.
a developerWorks mission and messaging article and work on creating and running a WebSphere MQ cluster provides specific guidelines for maintaining clusters.
Finally, the WebSphere MQ MustGather page contains troubleshooting sections for all major platforms and categorization by problem area.