HFS replies with 503 only when "overloaded", that means clients have crossed the limits you set. Try removing some limits.
If you have no limits, then that answer doesn't come from HFS.
Ensure the 503 comes truly from HFS. You can see it by this line in the reply headers
Server:HFS 2.3d
Sometimes people get replies from other systems (like proxies) and they don't even know.