Yeah i found the actual problem, i dont know why we did this
option tftp-server-name "xxx.xxx.xxx.xxx"
This renders your variable “next-server” more or less useless. 
Fixed it and its running like a charme.
And i have a update to the other issue (with the client) aswell. After reinstalling everything the hostname changer is working too 
So thank you very much for your support guys! If you have any questions please contact me
