I know this is a bit out of date now, but I found a cure for this error (in case anyone is still looking for a solution...)
I spoke to M-Audio tech support and they explained that unless I upgrade my OS I will keep getting these errors about not being able to access core midi.
It's something to do with OS 10.3x - if you upgrade to 10.4 the errors stop happening every time Apple release a security update. I don't know what happens with Leopard as I've not upgraded to that but 10.4 works OK
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