Try with manual IMAP commands instead:
Yes, sorry, I should have checked that before.
Does it return correct output?
Yes.
IIRC some clients (maybe Apple Mail?) had problems listing quota if quota root was empty.Hmpf. So I was just using a defective client to test my presumably defective confuration.
If that's the case, change it (quota = maildir:userquota).
fs:user did the trick. Thanks. Would this catch be worth being documented in the WIKI?