Gnus creates a new mail group automatically

[Hermann]

Hi,
usually all my incoming mails are stored in:
nnml+private:mail.misc
Since this evening some of my incoming mails are stored in:
nnfolder+archive:mail.misc
I created a group for outgoing mails called:
nnfolder+archive:ausgang
(Ausgang is German for outbox).
This is the only mail group with the backend nnfolder, and I didn't
change anything in my .gnus.
So what could be the reason for the automatic creation of the new
group/folder, and how can I avoid this in the future?
In addition: How to move the mails to my regular mail group
nnml+private:mail.misc?
Hermann