I've pinged the JGroups guys to see if they can help.
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The exception you're seeing is a bug in JGroups, I've created  to fix it. However, there is something else wrong for those many messages to get queued up...
As a workaround you can set TCP.use_send_queues=false.
I've spent a whole afternoon trying to reproduce the problem in various ways (including out of memory conditions), but I did not succeed - everything seems to work fine. I can only guess that the issue was due to the inhomogenous server environment or some kind of misconfiguration.
Anyway, thanks for your help and the pointer to the workaround if the problem arises another time.