All permissions granted?
Was it playing the Symphony from files on the Mac or were they on a NAS and were you playing that symphony through the Mac Speakers or via DLNA to the WiiM?
Is it maybe rebuilding thumbnails in the background? MC34 made a change to thumbnails where they'll need to be regenerated and cached. Go to Help > System Info... and see what it says under Processing and the thumbnails.
Anything unusual installed on the machine? Security software?
The Symphony music files are on a NAS (restored a version 33 library which worked fine) and sent to a WiiM via DLNA
It happened again today. Apple crash report attached