Is the same problem for all Synology admins. Different threads have as result the same. E.g.
https://forum.syncthing.net/t/panic-non-increasing-sequence/14570
At the end it was for me also the best to stop Syncthing, delete the content of the folder index-v0.14.0.db and start Syncthing again. This I make with all devices, it means Synologys, Windows, so it was clean. Since this point all is running without problems.
Also one productive NAS with Kastelo installation and update from v1.3.4 to now v1.4.0 works without any problems. The migration was without faults.