(solved in my case, see next post; but maybe still worth considering for upcoming main versions in case of incompatible protocols)
I have a sync group of 2 Windows PCs and an Android smartphone. When I installed Syncthing first, V0.11 was up to date, I loved it. One day I agreed to the update to V0.12 on one device, and though I had nodded away the “possible incompatibility” note, I didn’t expect it would result in mere ‘disconnected’ status on all devices, with not a trace of a helping message. I was unable to use Syncthing for several weeks and really felt relief, when I got V0.12 working again.
Now I noticed that (as the default setting?) on one PC the “auto update” mark was set, after again several days of, to me, unexplainable connection problems.
So I’d suggest that preferredly a synch group of devices should only do upgrades after they negotiated that each device has the new version ready on board; or at least the disconnected status display should be enhanced to show a version incompatibility note or the version of the remote device; and in this case, an easier update undo than to open the folder, delete syncthing.exe and rename syncthing.exe.old should be available. As I interpret the user statistics when new versions come up, my issue applies to several people.