To be clear, I consider this a problem with macOS and not Syncthing. I expect Apple to resolve it if you report it to them, as you should as a beta tester. There is nothing we can do from Synching side at the moment, posting logs here doesnāt help.
I have no problem an a 64bit Ubuntu 14.04 machine with the same version of Syncthing.
Maybe the fix in the Go language hasnāt reached the arm version yet?
I have not tested that exactly build, but homebrew golang-1.6.3 / syncthing-0.14.2 work fine for me the first time on Sierra Beta (16A254g). Earlier versions did not (not tested all of them).
Go 1.4 was not updated yet, so bootstrapping with homebrew on macOS Sierra beta 4 / public beta 3 fails. Currently I donāt know a simple/automated way to build / run Syncthing on macOS Sierra, but the Go team promised to update 1.4 when Apple released the final Sierra version.
I meant automated building HEAD with homebrew (brew install syncthing --HEAD) fails because it wants to bootstrap go.
[edit] I see you can skip dependencies with --ignore-dependencies, so manually installing Go (donāt forget to setup $GOPATH) and it should work with homebrew again.
[edit2] Looks like it is not that easy, homebrew likes to keep things separate for good reasons and ignores the user specific environment (./build.sh: line 14: go: command not found). Back to checkout/compile by hand. Itās a beta after all, not complaining.