I'd like to answer this a bit broader, because the subject of this question can be found via a search engine:
--exclude-directories=list expects absolute paths [1]. This means with host.org/fu/bar/ you have to write --exclude-directories=/fu/bar.
This can be a problem, if you always want to exclude a folder with a specific name, no matter where it is exactly (for example a 'thumbs' folder).
For this we can use --reject-regex [2] like this: --reject-regex="/thumbs/". Given this is now regex and not a comma-separated string list, we can exclude multiple folders via regex1|regex2|regex3: --reject-regex="/thumbs/|/css/". Keep in mind that certain characters like . have a special meaning in regex and need to be escaped to be part of a folder name: "/\.svn/".