We are updating our stash instances and used the backup-client to save all our data before proceeding. The backup failed at the database backup step and just stopped running. Our stash instance is now locked. We tried the following:

1. Unlocking it via the Web interface, but, as described in STASH-5182, this does not work.
2. Use the REST maintenance API to manually unlock the instance, but we got a 404 status code while specifying the given token.

How can we unlock the stash instance? We are neither able to connect to its Web interface, nor to start another backup (the "lock" request seems to be running forever).