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Possible errors

First of all, try rebuild-index command. It will take some time but it will probably fix things. If it still gives this warning, consider creating a bug report, since Restic should not "lose" the data. Anyway you can get rid of the error if you can give up the snapshots which are effected by this:

$ restic find --tree XXXXX
Found tree 453f9dd1999c9bd89cfd5dd3e574216b6cc0e247da5656ea7d7a19461aeef447
 ... path /some/path
 ... in snapshot br23e8943b (2020-01-22 06:27:20)
$ restic forget br23e8943b