1. My mess

root@dc:~# samba-tool spn list srv11$
srv11$
User CN=srv11,CN=Computers,DC=openforce,DC=org has the following servicePrincipalName:
     HOST/SRV11
     HOST/srv11.openforce.org
     LDAP/srv11.openforce.org@OPENFORCE.ORG
     ldap/srv11.openforce.org/OPENFORCE
     ldap/srv11.openforce.org/OPENFORCE.ORG
     LDAP/SRV11

2. After clean up

root@dc:~# samba-tool spn add LDAP/srv11.openforce.org srv11$
root@dc:~# samba-tool spn delete ldap/srv11.openforce.org/OPENFORCE.ORG srv11$
root@dc:~# samba-tool spn delete ldap/srv11.openforce.org/OPENFORCE srv11$
root@dc:~# samba-tool spn delete LDAP/srv11.openforce.org@OPENFORCE.ORG srv11$
root@dc:~# samba-tool spn list srv11$
srv11$
User CN=srv11,CN=Computers,DC=openforce,DC=org has the following servicePrincipalName:
     HOST/SRV11
     HOST/srv11.openforce.org
     LDAP/SRV11
     LDAP/srv11.openforce.org

3. Now it works:

root@dc:~# samba-tool domain exportkeytab ldap-srv11.keytab --principal=host/srv11.openforce.org@OPENFORCE.ORG
root@dc:~# samba-tool domain exportkeytab ldap-srv11.keytab --principal=ldap/srv11.openforce.org@OPENFORCE.ORG
root@dc:~#