解决了， 不是思源的问题。

我查看了日志

I 2021/11/30 21:13:13 sync.go:85: sync before boot
E 2021/11/30 21:13:14 rsync.go:35: copy [C:\Users\xxxx\Documents\SiYuan\incremental\sync-encrypt] to [C:\Users\xxxx\Documents\SiYuan\sync] failed: exit status 23 rsync: [generator] failed to set times on "/cygdrive/C/Users/xxxx/Documents/SiYuan/sync/.siyuan": Is a directory (21)
rsync: [receiver] mkstemp "/cygdrive/C/Users/xxxx/Documents/SiYuan/sync/.siyuan/.conf.json.67Hodf" failed: Permission denied (13)

应该是下午用杀毒软件全盘检测后，出现权限错误。将 c:\User\xxxx\Documents\SiYuan\sync 目录删除后再启动，就解决了。

我还不知道怎么使用第三方同步盘。我就是今天升了个级，没有进行其它操作。