kai11,
Checkpoint recommend that Smartcenter server is always upgarded first so you should never have a newer release on on your enforcement modules than on the smartcenter server. see:-
http://updates.checkpoint.com/filese...gradeGuide.pdf "The highest version i can select on Smartcenter - R55 for the Gateway-object is "NG with AI"" - I suspect this is because your smartcenter server is not NGX 'aware' as it's running at NG R55.
Hope this helps,