That took quite some time to post… I hope someone can look at it and give me a nudge in the right direction… thanks again!
I have been reading up on open files and found this…
"The too many open files error is normally because you have too many open sockets (Sockets consume file handles the same as open files).
In this case you have hit the maximum number of incoming client connections, which is probably the default 1024 limit."
a) I am wondering is some of my settings are not suitable e.g. I have set QOS to 2.
b) also, I’m thinking to upgrade the clients on the 2x Pis running the older versions to the same version, as I suspect that will be recommended… I’ll hold off until I hear something, in case it’s better to test the setup as it is