save failed to open mailbox quarantine

[Eggert Ehmke FTV Spandau]

I have dovecot 2.2.13 set up with spamassassin and amavis. This works fine so 
far, Spam emails are forwarded to "username+Quarantine at 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 at mydomain.de): Info: 
msgid=<2707212.2aHC3KDWrN at wookie>: save failed to open mailbox quarantine: 
Mailbox doesn't exist: quarantine
Nov 14 14:16:13 lda(username at mydomain.de): Info: 
msgid=<2707212.2aHC3KDWrN at 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?