this is a known issue, made it's appearance with macOS Sierra beta1. This is the public Bug Report which already made it's way to Apple Inc.:
Some previous reports were sent in directly to TS under Apple Inc. NDA in June.
Running a "Recover file …" on any of a FMP File resolved the issue for me.
I can confirm efficientbizz information is correct.
I am also going to move this thread from the FileMaker Community Feedback Space, which is specifically for input on the Community itself, to the Discussions Space where you should receive more views and potentially more advice on this topic!