Gnus fails to reconnect to IMAP server after idle timeout

[Kai Großjohann]

After some idle time, my IMAP server (Cyrus IMAP4
v2.1.18-IPv6-Debian-2.1.18-1+sarge2) times out connections.  (At least
that's what it looks like: In the echo area in Emacs I see something
along the lines of "garbage" and "BYE idle too long".)

Gnus notices that the server isn't responding, but it puts the server
in the denied state and thus doesn't reconnect properly.  Closing and
then reopening the server works, but is a hassle.

What can I do to help Gnus reconnect?

Kai