Unrecognized backend "file" status after deployment

After deployment of charm-duplicity using a juju bundle with this config:

[...]
  duplicity:
    charm: ch:bootstack-charmers-duplicity
    num_units: 0
    options:
      backend: "file"
      disable_encryption: true
[...]

Unit has entered blocked/idle state:

duplicity/0* blocked idle Unrecognized backend "file"

Debug log of the unit:
https://pastebin.ubuntu.com/p/7HcVFmY2MB/

Versions: charm revision 2, channel stable.

The workaround is to unset and set backend="file" again:

$ juju config duplicity backend=""
$ juju config duplicity backend="file"

duplicity/0* active idle 10.5.0.94 Ready

Running the same without clearing this config option first results in:
$ juju config duplicity backend="file"
WARNING the configuration setting "backend" already has the value "file"

And the status doesn't get cleared.

I managed to reproduce it with both bionic and focal on the primary unit.