When running a backup with stash-diy-backup, I keep getting a "ERROR: Locking this Stash instance failed" error

When running a backup with stash-diy-backup (./stash.diy-backup.sh), I keep getting a the following error, after the repo backup completes (error on last line):

stash/tmp/webresources/4.cachedfile
sent 2.87G bytes received 67.11K bytes 34.78M bytes/sec
total size is 2.87G speedup is 1.00
[localhost] INFO: Prepared backup of /var/atlassian/application-data/stash to /var/atlassian/application-data/stash-backup/stash-home/
[localhost] ERROR: Locking this Stash instance failed