I just had a problem where none of my Syncthing nodes would connect, even after restarting all but one. Once I restarted the last node the other nodes found each other without restarting, and after last node finished restarting it connected too.
The last node is running syncthing 0.13.7 on Synology. My other nodes are two Windows machines running 0.13.7 and one Synology machine with syncthing 0.13.4. Both Synology’s are ARM, but different models. I am using both dynamic and static addresses, because the Synology’s are on different networks and the Windows machines are laptops that connect locally to one Synology and remotely to the other Synology, based on their location.
I don’t have much time now so I thought I would at least put this experience out here on the forums. I’ve just upgraded to version 13 yesterday, so I hope it was just a fluke and it doesn’t happen again. Although there seems to be two issues here 1) there was a problem on one node and 2) the other nodes can get stuck if one of the other nodes have an issue.