Linux: copied one user account to another and now I can't access the browser UI

It might be worth my adding the following. Just now, after (coincidentally?) upgrading syncthing, I had the problem again. To circumvent the problem, I had to remove the entire password text from /home/<user>/.config/syncthing/config.xml, i.e. all of <password>text_text</password> and restart syncthing (sudo killall syncthing and then syncthing).