We can always count on new bugs with Apple these days. Every software update is a risk, like opening a can of badly dented food.... will it have botulism or not?
Working for many months now, Thunderbolt networking between the MPG Macs is now inoperable in macOS 10.11.6. No number of reboots or restarting file sharing fixes it.
Such a 'helpful' message! I am the system administrator and it all worked fine before OS X 10.11.6.