The CLI help lists the Rackspace Cloud files protocol as

$ duck --help Usage:duck [options...] ... Default protocols ...

Rackspace Cloud Files (US) ​rackspace://<container>/<key>

...

However the documentation at ​https://trac.cyberduck.io/wiki/help/en/howto/cli#URI lists the protocol as rackspace:/<container>/<key> (absolute) rackspace:<container>/<key> (relative)

It appears that some time in the last few months prior to 6.9.4, ​rackspace:// was treated the same as rackspace:/ but this is no longer the case the the URI cannot be specified with two slashes. If it is the <container> item is treated as server address which is non-sensical for the rackspace protocol.

To resolve this bug the help should have the extra slash removed to become:

$ duck --help Usage:duck [options...] ... Default protocols ...

Rackspace Cloud Files (US) rackspace:/<container>/<key>

...

I have examined the source and each protocol is enumerated from lines 59-71 of ​https://github.com/iterate-ch/cyberduck/blob/master/cli/src/main/java/ch/cyberduck/cli/TerminalHelpPrinter.java