cannot boot: ZFS: unsupported feature" com.klarasystems:vdev_zaps_v2

Hi,
 
Quite unexpected (after a portupgrade) I suddenly can't boot anymore:
 
[...]
BIOS drive C: is disk0
ZFS: unsupported feature: com.klarasystems:vdev_zaps_v2
 
Can't find /boot/zfsloader
 
Can't find /boot/loader
 
Can't find /boot/kernel/kernel
 
FreeBSD/x86 boot
Default: disk-1:/boot/kernel/kernel
boot:
[...]
 
And it just sits there.
I have no idea what to do from here.
Does anyone know what to do next?
 
Thanks in advance,
Regards,
Marco