Unable to retrieve folder revisions through /metadata using rev parameter.

Thanks for the detailed post, and sorry the documentation isn't clearer on this. Folders do technically have rev values (due to the specifics of how this works on the backend) but /revisions and /restore only work with files, not folders.

Likewise, /metadata with a rev parameter isn't meant to be used for folders, and usually trying to do so gives the more helpful error:

Directories can not be used with the rev parameter

Unfortunately though, in certain cases, you'll get the "Invalid or outdated rev" error message instead.