Thanks, that is helpful. I deleted a lot of files on my harddisk, now syncthing has synchronized both systems.
Now one of the two systems reports “up to date”, the other one reports “out of sync” and lists 68 files. What can I do to get the status “up to date” on this system, too? I don’t need the mentioned 68 files any more, but it would be a lot of work to delete them by hand.