I know this issue arises with MAC-users, but the error “normalizing path: item is not in the correct UTF8 normalization form” happens to me in pure linux-environments as well …
All Systems are opensuse LEAP15 running either ext4 or xfs as filesystems
As soons as those letters (öäüß) happen in filenames - syncthing fails
Then those filenames come from a system with a different normalization, typically a Mac, and renaming them is the solution. (Or if they don’t, you need to figure out why they’re named according to a different standard than that in use on your system.)