I've looked through the archive and it seems there have been similar situations but no solutions.
FMServer 18.104.22.1688 running on development license
2016 MacBook Pro w/ Touch Bar on macOS Sierra 10.12.3 (16D32)
Both are on the same machine. The client gets disconnected, it seems, at random.
There are two things which are non-standard.
1) The laptop is connected to the Ethernet network using a Belkin USB-C to Ethernet adapter. This connection is being shared so my iPad and iPhone can get online.
2) I'm running MirrorSync on the machine as well.