Just to document this, @tosch explains in Docker-Swarm & NFS4-Volume gives [Errno 9] Bad file descriptor - #4 by tosch :
Liveproxyd tries to create the file
~site/tmp/run/livewhich needs to be a FIFO pipe and fails i assume.
This tells me what I was hoping to get OMD to tell me at least. I’m at least now able to tackle the underlying problem (thanks @tosch), but I still see problems in OMD here. I’m not really sure “go wade through thousands of lines of Python code” is a particularly viable solution, although it is one I may resort to in future.