I use pulseaudio version 4.0-2 on archlinux. When the module-combine-sink is loaded and I try to disable one of my two soundcards in pavucontrol pulseaudio stops with this error message: module-combine-sink.c: Assertion 'pa_idxset_remove_by_data(o->userdata->outputs, o, NULL)' failed at modules/module-combine-sink.c:927, function on output_free(). Aborting.
Thanks for the report! I sent a fix to the mailing list: http://thread.gmane.org/gmane.comp.audio.pulseaudio.general/17242
Fixed: http://cgit.freedesktop.org/pulseaudio/pulseaudio/commit/?id=0e9baa429ea2620dbb11f4b3a26eaa46620a9124
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