what can be the reason when a new kernel does not boot ?
As a paranoid admin I use crypted swap partitions, so my crypttab contains something like that:

swap /dev/sda2 /dev/random swap

Thats all right, nothing wrong with that. But why did the server not boot on recent kernels ?

It seems that in recent kernel versions there is not enough entropy available when crypttab early disks are run and as it is a server, no keyboard/mouse etc creates one. So, what happens: nothing. The server waits until enough entropy is available, which seems like forever…

Changing to /dev/urandom btw works, but is of course lesser save.