save failed to open mailbox quarantine

I have dovecot 2.2.13 set up with spamassassin and amavis. This works fine so far, Spam emails are forwarded to "username+Quarantine@mydomain.de". The recipient_delimiter=+ is set, the Quarantine folder exists. However, Dovecot tries to deliver it to a folder quarantine, that does not exist. In the dovecot-deliver.log there are these lines:

Nov 14 14:16:13 lda(username@mydomain.de): Info: msgid=2707212.2aHC3KDWrN@wookie: save failed to open mailbox quarantine: Mailbox doesn't exist: quarantine Nov 14 14:16:13 lda(username@mydomain.de): Info: msgid=2707212.2aHC3KDWrN@xxxxxx: saved mail to INBOX

Indeed the message is found in the inbox. What can be done so the folder name is not converted to lower case?