gnus-cloud and message disappearing on my imap server (gnus only)

[Announcements and discussions for GNUS, the GNU Emacs Usenet newsreader (in English)]

Out of curiosity I tried to give gnus-cloud a try:

So I added 

(require 'gnus-cloud)

(setq gnus-cloud-method 'nnimap
      gnus-cloud-server "imap.gmail.com"
      gnus-cloud-user-name "o...@ucm.es")

Read some messages

After closing gnus, starting emacs and gnus, these messages are no longer 
displayed, although they are still on the server (reading with the google 
webinterface or the iPhone).

Any idea what happened, and more importantely how to display them again. I am 
rather annoyed I have to admit.

Uwe Brauer