After 5 hours and many tries I could made updates on all machines - 2 of the 4 servers were finished already. The both remaining are scanning now.
What I don´t understand - why are the definitions in the iso outdated this much? Other live cd´s are updated on a daily base. It happened already in the past years, that I had to scan machines wich are not online with a stick or disc I prepared the day before I head to the customer.
But anyway, patience was the key, thank you all for the replies.