As I see it, the only way that [packOSC] can receive a message before it has completed the previous message is if its own output triggers another message to its input. It may be possible to detect that special case and either let it happen or not, but it's tricky because the buffer for the list is deallocated as soon as the list is output, and the new message arrives while that is taking place. Possibly using a fixed buffer would work better, and also speed up the code. The function then might just exit before it is called again, avoiding the error.
Martin