The problem:

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$ zypper ref
Retrieving repository 'SLES11-Extras' metadata [|]
Signature verification failed for file 'repomd.xml' from repository 'SLES11-Extras'.
Warning: This might be caused by a malicious change in the file!
Continuing might be risky. Continue anyway? [yes/no] (no): yes
 
Warning: Digest verification failed for file 'updateinfo.xml.gz'
[/var/cache/zypp/raw/nu_novell_com:SLES11-ExtrasdljTMy/repodata/updateinfo.xml.gz]
 
  expected 8a45de1f10c250727961e42d2d2503244430404d
  but got  88197abfd21b342c18aecf8a2cb5c57853f080d3
 
Accepting packages with wrong checksums can lead to a corrupted system and in extreme cases even to a system compromise.
 
However if you made certain that the file with checksum '8819..' is secure, correct
and should be used within this operation, enter the first 4 characters of the checksum
to unblock using this file on your own risk. Empty input will discard the file.
 
Unblock or discard? [8819/? shows all options] (discard):
The solution:
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zypper clean -a
Now rerun zypper and the access to the repo should not show up the initial error.