restic: Unable to remove lock from repo on Synology NAS

What's the version of restic you used? 

restic 0.9.6 compiled with go1.12.12 on linux/amd64

What commands did you execute to get to where the bug occurred?

RESTIC_PASSWORD=my-password restic -r sftp://t440_backup@nas.lan/t440_backup/restic snapshots

What did you expect?

A correct return to shell prompt after snapshots listed.

What happened instead?

After snapshots listed the error is reported infinitelly (until I break it with Ctrl-C - a lot of Ctrl-C was needed):

------------------------------------------------------------
17 snapshots
Remove(<lock/f845632e7c>) returned error, retrying after 615.202403ms: file does not exist
Remove(<lock/f845632e7c>) returned error, retrying after 931.520507ms: file does not exist
...last line repeating...

Are you aware of a way to reproduce the bug?

Yes, it occurs every time I access repo on Synology. Until the repo was on Debian Linux I have never met this error. I tried to save log (export DEBUG_LOG=...) but no logfile was created.

State of lock file on NAS looks good: file exists, permissions are ok.

bash-4.4# pwd
/volume1/t440_backup/restic

bash-4.4# ls -l
total 4
-rw------- 1 t440_backup users  155 Nov 28  2021 config
drwx------ 1 t440_backup users 1024 Nov 28  2021 data
drwx------ 1 t440_backup users  640 Aug 24 04:35 index
drwx------ 1 t440_backup users  128 Nov 28  2021 keys
drwx------ 1 t440_backup users  128 Aug 25 10:18 locks
drwx------ 1 t440_backup users 2176 Jul 24 17:33 snapshots

bash-4.4# ls -l locks
total 4
-rw------- 1 t440_backup users 166 Aug 25 10:18 f845632e7c51a874d1cfbe419b3016c062920089370890fb98b9ec2cf578d1af