Nextcloud's file operations API apparently is unable to proberly deal with relative paths (even though the docs tell us otherwise). It (a) performs file permission checks on the base directory rather than the respective parent directory (:confused:), and (b) blocks relative paths like '..' (likely as a security measure - by using the most unsophisticated approach :unamused:). Also see nextcloud/server#26396. We better do this on our own...
Fixes #141
Signed-off-by: Daniel Rudolf <github.com@daniel-rudolf.de>