Gnus configuration to connect IMAP server?

[Rolf Ade <rolf@pointsman.de>]

I'm using emacs 24.5.1 on linux.

Up to now I used gnus just for an nntp feed and, as
gnus-secondary-select-methods, gmane.

I'm trying to add an IMAP account as another
gnus-secondary-select-methods.

The IMAP server is working; I'm able to access the IMAP account with
thunderbird.

I've tried all kind of configurations, starting from simple up to
spelling out all kind of parameters, but no fun. E.g.

(setq gnus-secondary-select-methods '((nntp "news.gmane.org")
                                      (nnimap "umkehrbar"
                                              (nnimap-stream starttls)
                                              (nnimap-address "xxx.de")
                                              (nnimap-user "yyy@xxx.de")
                                              (nnimap-inbox "INBOX"))))

The *Messages* buffer tells me things like 

"Opening nnimap server on umkehrbar...failed: BAD Invalid characters in atom"

that doesn't help much.

I'm able to talk to the IMAP server on the cmd line:

rolf@linux-qg7d:~> gnutls-cli -s -p imap xxx.de
Processed 149 CA certificate(s).
Resolving 'xxx.de'...
Connecting to 'ddd.ddd.ddd.ddd:143'...
|<1>| Note that the security level of the Diffie-Hellman key exchange has been lowered to 512 bits and this may allow decryption of the session data

- Simple Client Mode:

* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE STARTTLS AUTH=PLAIN AUTH=LOGIN AUTH=DIGEST-MD5 AUTH=CRAM-MD5] Dovecot ready.
a1 LOGIN yyy@xxx.de zzz
a1 OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE QUOTA] Logged in
a2 LIST "" "*"
* LIST (\HasChildren) "." INBOX
* LIST (\HasNoChildren) "." INBOX.Spam
* LIST (\HasNoChildren) "." INBOX.Trash
* LIST (\HasNoChildren) "." INBOX.Sent
* LIST (\HasNoChildren) "." INBOX.Drafts
a2 OK List completed (0.000 secs).
a3 EXAMINE INBOX
* FLAGS (\Answered \Flagged \Deleted \Seen \Draft NonJunk)
* OK [PERMANENTFLAGS ()] Read-only mailbox.
* 4 EXISTS
* 0 RECENT
* OK [UIDVALIDITY 1470408319] UIDs valid
* OK [UIDNEXT 5] Predicted next UID
* OK [HIGHESTMODSEQ 12] Highest
a3 OK [READ-ONLY] Examine completed (0.000 secs).
a4 LOGOUT
* BYE Logging out
a4 OK Logout completed.
- Peer has closed the GnuTLS connection

What can I do to debug the problem?

Thanks
rolf