Acer C720 crash at boot

Hi,

I can no longer boot my Acer C720 with current; during boot I get
a panic and the system drops into kdb, unfortunately without a
working keyboard. What is visible on the screen looks like
it might be a stack trace; top line starts with vpanic().
Would transcribing the screen output be of any use?

When I first noticed this a few days ago, I assumed I might
have just updated src in the mittle of a change, so I went back
to kernel.old and updated src again yesterday (twice within
some time to make sure I'm not in the middle of something)
and rebuilt. kernel.old is a few weeks old now.
I'm using GENERIC.

Wolfgang