Comment 1 for bug 1830204

Indeed, according to the documentation (https://www.percona.com/doc/percona-xtrabackup) the
--incremental=basedir=BASEDIR is needed and should point to the last full back-up that was done.

This would mean the action finding the most recent full backup from the currently passed basedir (in the action) and using that (in all probability). Alternatively, the user would need to specify the incremental basedir (which would mean inspecting the 'basedir' that had previously been used in the last full backup action) to be able to pass that value. And in order to do that, an action would be needed (probably) to list the full backup base dirs relative to a passed 'root basedir' that is used in the full backup.

Which means, this is a bit more complicated than a simple bug fix; I recommend a spec be written to propose how the feature of incremental backups be handled, and perhaps, in the meantime, remove the incremental option as it doesn't work.