It seems to be anything which isn't either ASCII or UTF-8. My father's PC had files written by old versions of OpenOffice (when it was called StarOffice), with "native" names but before his PC was set up to use UTF-8. Early versions of Syncthing didn't have problems with those files, but later versions refused to back them up. I had to rename all those files one by one to get it working again.

If you ever have to do to this again checkout convmv - this can convert for example latin1 encoded file names into utf8 encoded file names.

wish I'd known about this sooner - thanks