Sorry if here isn't right place for asking this question

I have a ltsp server ( edubuntu 12.04 ) it was working fine. With any try for connecting to
server, client received next ip address ( means first time and next time
same client ). suddenly on I got pxe-e11 error in connecting.
Nothing changed in my network. The real problem is after error happens for first time
changing ip address or even connet a PC with cross cable to server I still got the error. If I
replace the backup ( it is exact same as the server ) it works fine again till .

Can anyone help on this please