Issue with doveadm backup

[Asai]

Greetings,

Lately I've been seeing this error pop up when running a nightly cron to 
a backup share with doveadm:

 Error: stat(/vmail/domain.tld/user/.dovecot-box-sync.lock/tmp) failed: Not a 
directory: 1 Time(s)

This is the command:

doveadm backup -u u...@domain.tld maildir:backupshare/daily/domain.tld/user

Our main vmail directory is on an NFS share and seems to be working 
fine.  This has only occurred recently.

Can anyone offer any insights into what this is and fixing it if need be?

--
Asai