That is a new requirement for Duplicati: mandatory API password.
With Docker, you can set the environment variable:
DUPLICATI__WEBSERVICE_PASSWORD=<your password>
This is caused by the update to .NET8 where Linux advisory file locking is now honored. The real fix is to use LVM and set up snapshots so you get a consistent view, but you can also choose to get a “write-consistent” view as Duplicati did in 2.0.8.1 and older.
To get a “write-consistent” view, you can set the option --ignore-advisory-locking=true (it can be set globally to work for all backups).
with Docker, you can also consider setting the environment variable:
DOTNET_SYSTEM_IO_DISABLEFILELOCKING=true
Which will ignore any advisory locks from within Duplicati.
I have created a documentation page that describes some common setups for Duplicati in Docker.