Vine Server no screenshot without attached display 10.6.8

first post…
hope you all are doing well !

Problem :
I worked with Vine Server 3.0 configured to launch at boot
Macmini1,1, OS 10.4.11 with no display attached
all fine.

Now with the same mac but OS 10.6.8,
Vine Server 3.11
no display attached,
everything seems to work but
screenshot is impossible on the Vine Server - mac
not even with an attached keyboard …
Is there maybe a problem using 10.6.8 windowserver without a display attached ?
If a display is attached, it works

Thank you for help

and best regards