Frequent error: unknown namespace Trash

[Michael Ludwig]

Hello to all of you! ;-)

My first post on this list. I frequently get this error in dovecot.log
every time a user logs in and out:

=============================
imap(mailaddress): Error: quota: Unknown namespace: Trash
=============================


When I change my conf.d/90-quota.conf from "Trash" to ".Trash" (as this is
the imap/maildir folder name, the error stays the same, but with the
".Trash" in dovecot.log
This is what my 90-quoato.conf looks like:

=============================
plugin {
  quota_rule = *:storage=2G
  quota_rule2 = Trash:storage=+100M
  quota_grace = 10%%
}
=============================

Where can I start my search, what is going wrong?
Just for letting you know: This mailserver is a fresh and clean setup.
I am using postfix/dovecotvimbadmin/mysql/spamassassin/clamsmtp.

Cheers,
Michael