Easy to reproduce

$ cd $HOME/Sync/data
$ ln -snf /link-a-non-existing-file to-here

or from any sub folders inside your sync path

$ cd $HOME/Sync/data/ebooks
$ ln -snf /usr/local/www/non-existing book.pdf

then you will see the error message from your browser.

Filesystem Watcher Errors: error while traversing /path/to/exists/file_or_dir: no such file or directory

Version Information

v0.14.54, FreeBSD (64 bit)

Syncthing Version: v0.14.54
OS Version: FreeBSD 12.0-RC2
Browser Version: Chrome Version 70.0.3538.67 (Official Build) (64-bit)