My client sent me a screen shot of a message. It reads "The installed version of Server is no longer supported on this version of OS X." It goes on to say, "The services have been stopped and service data is preserved, visit the Mac App Store to get the latest version of Server."
So he's running a Mac Air out in the field. He's running server on a Mac Mini back at his office, with Mac OS High Sierra (the latest version) and FileMaker Server 16, there. As far as I know he's also running FileMaker Pro 16 on his Mac Air. Is it possible that he somehow has an old version of server installed on his Mac Air (which is clearly not needed)? And it's further possible that he ran some kind of software update from Apple's App Store that triggered the "incompatibility" and thus the message?
He says that FileMaker (Pro, I'm assuming) is slow and crashes.