I recovered my data.
And we owe some pizzas/beers/rawjuicedrinks to the LVM designer who
insisted on human readable metadata. It has been invaluable in many
data recovery scenarios.
Thanks for all your help.
The problem was in fact knoppix, but not because it had an
older version of lvm2 or even lvm1.
The problem was that the human-readable part I extracted from
the beginning of the device contained invisible characters in
front of the
'vol0' label in the file 'volt.txt' .
These characters were not visible in terminal using 'cat',
'more', and they also did not appear in standard error as a
consequence of the 'vgcfgrestore' command, as witnessed by the
first line of the error: